MEMS display apparatus

ABSTRACT

This invention relates to display apparatuses having an array of light modulators and a plurality of spacers distributed within the interior of the array. The display apparatus may also include a reflective aperture layer disposed on a front facing surface of a substrate included in the display apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 11/656,307, entitled “Methods and Apparatus for Actuating Displays” and filed Jan. 19, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 11/251,035, entitled “Methods and Apparatus for Actuating Displays” and filed Oct. 14, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 11/218,690, entitled “Methods and Apparatus for Spatial Light Modulation” and filed Sep. 2, 2005, the latter two of which claim priority to and benefit of, U.S. Provisional Patent Application No. 60/676,053, entitled “MEMS Based Optical Display” and filed on Apr. 29, 2005; and U.S. Provisional Patent Application No. 60/655,827, entitled MEMS Based Optical Display Modules” and filed on Feb. 23, 2005. The entirety of each of these provisional and non-provisional applications is incorporated by reference herein.

FIELD OF THE INVENTION

In general, the invention relates to the field of video displays, in particular, the invention relates to mechanically actuated display apparatus.

BACKGROUND OF THE INVENTION

Displays built from mechanical light modulators are an attractive alternative to displays based on liquid crystal technology. Mechanical light modulators are fast enough to display video content with good viewing angles and with a wide range of color and grey scale. Mechanical light modulators have been successful in projection display applications. Backlit displays using mechanical light modulators have not yet demonstrated sufficiently attractive combinations of brightness and low power. There is a need in the art for fast, bright, low-powered mechanically actuated displays. Specifically there is a need for mechanically actuated displays that include bi-stable mechanisms and that can be driven at low voltages for reduced power consumption.

SUMMARY OF THE INVENTION

According to one aspect of the invention, the display apparatus includes a first substrate, a second substrate, an array of MEMS light modulators, and a spacer. The array of MEMS light modulators are formed on one of the first and second substrates. The spacer is located within the interior of the array, integrally formed from or connected to the first substrate at a first end and connected to a second substrate at a second end. The connection of the spacer to one of the first and second substrates may include a connection to a stack of at least one thin film deposited on the substrate. The thin film may include one of a reflective aperture layer, a light absorbing layer, and a color filter.

The spacer may be etched from the first substrate and/or from a film deposited on the first substrate. The spacer may form an electrical connection between an electrical component on the first substrate to an electrical component on the second substrate. The spacer may be formed from a polymer, a metal and/or an insulative material. The spacer may interfit with an aligning element formed on the second substrate. The spacer may be between about 1 micron and about 10 microns tall.

The first substrate and the second substrate may both be substantially rigid. Alternatively, the first and second substrates may both be substantially flexible. Or, one of the first substrate and the second substrate is substantially rigid, and the other of the first substrate and the second substrate is substantially flexible. At least one of the first substrate and the second substrate is substantially transparent.

The first substrate or the second substrate may include a front surface of a display device. A light guide may be distinct from the first and second substrates. The spacer and a plurality of additional spacers may be positioned within the array with a spacer density less than or equal to one spacer per four light modulators. The MEMS light modulators may correspond to respective display pixels, where each of the respective display pixels includes at least one spacer. A space between the first and second substrates may form a gap, which may be filled with fluid, such as a lubricant.

The MEMS light modulators may be configured to selectively obstruct the passage of light and may include shutter-based light modulators and/or electrowetting-based light modulators. The MEMS light modulators may selectively extract light from a guide. The spacer may limit a range of motion of a component in one of the MEMS light modulators.

According to another aspect of the invention, a display apparatus includes a first substrate, a reflective aperture layer, and a plurality of MEMS light modulators. The first substrate has a front-facing surface and a rear-facing surface. The reflective aperture layer includes a plurality of apertures disposed on the front-facing surface of the first substrate. The plurality of MEMS light modulators modulate light directed towards the plurality of apertures to form an image.

The first substrate may include a light guide. Alternatively, a light guide may be positioned behind the first substrate, where the reflective aperture layer may reflect light not passing through the plurality of apertures back towards the light guide. In this case, the light guide may be in intimate contact with the first substrate.

The plurality of MEMS light modulators may include shutter-based light modulators and/or electrowetting light modulators. An active matrix control matrix may control the plurality of MEMS light modulators, where the active matrix may include at least one switch corresponding to respective MEMS light modulators. The first substrate may be transparent. The reflective aperture layer may be formed from one of a mirror, a dielectric mirror, and a metallic film.

The MEMS light modulators may be disposed on the first substrate. The first substrate may be positioned such that the reflective aperture layer is proximate the plurality of light modulators. The plurality of apertures may correspond to respective light modulators of the plurality of light modulators.

In one embodiment, the first substrate is positioned with respect to a light guide so as to form a gap between the first substrate and the light guide. A fluid, which may be air, may fill the gap. The fluid may have a first index of refraction and the light guide may have a second index of refraction, where the first index of refraction is less than the second index of refraction.

In another embodiment, a second substrate is positioned in front of the front-facing surface of the first substrate. The plurality of MEMS light modulators may be formed on the second substrate or on a rear-facing surface of the second substrate. A control matrix may be formed on the second substrate for controlling the plurality of MEMS light modulators. The second substrate may be transparent. Mechanically interlocking features and/or an adhesive may maintain lateral alignment between the first and second substrates and may restrict relative lateral movement of the first and second substrates to less than 5 microns in any dimension. The first substrate may be positioned between the second substrate and a light guide.

The first substrate may be positioned with respect to the second substrate so as to form a gap between the first substrate and the second substrate. The gap may be maintained by spacers and/or filled with a liquid or fluid, such as a lubricant. The fluid may have a first index of refraction and the substrate may have a second index of refraction, where the first index of refraction is greater than or substantially equal to that of the second index of refraction.

BRIEF DESCRIPTION OF THE DRAWINGS

The system and methods may be better understood from the following illustrative description with reference to the following drawings in which:

FIG. 1 is conceptual isometric view of a display apparatus, according to an illustrative embodiment of the invention;

FIGS. 2A-2B are top views of dual compliant beam electrode actuator-based shutter assemblies for use in a display apparatus, according to an illustrative embodiment of the invention;

FIG. 3A is a diagram illustrating various compliant electrode shapes suitable for inclusion in dual compliant electrode actuator-based shutter assemblies;

FIG. 3B is a diagram illustrating the incremental energy needed to move dual compliant electrode actuator-based shutter assemblies having the shapes illustrated in FIG. 3A;

FIGS. 3C-3F are top views of the compliant beam electrode actuator-based shutter assembly of FIG. 2A in various stages of actuation;

FIGS. 4A and 4B are cross section views of a dual compliant electrode actuator-based mirror-based light modulator in an active and an inactive state, according to an illustrative embodiment of the invention;

FIG. 5 is a top view of a dual compliant beam electrode actuator-based shutter assembly having a beam with thickness which varies along its length, according to an illustrative embodiment of the invention;

FIG. 6 is an isometric view of a dual compliant beam electrode actuator-based shutter assembly, according to an illustrative embodiment of the invention;

FIG. 7 is a top view of a dual compliant beam electrode actuator-based shutter assembly including a return spring, according to an illustrative embodiment of the invention;

FIG. 8 is a top view of a dual compliant beam electrode actuator-based shutter assembly having separate open and close actuators, according to an illustrative embodiment of the invention;

FIG. 9 is a conceptual diagram of an active matrix array for controlling dual compliant electrode actuator based-light modulators, according to an illustrative embodiment of the invention;

FIG. 10 is a conceptual diagram of a second active matrix array for controlling dual compliant electrode actuator based-light modulators, according to an illustrative embodiment of the invention;

FIG. 11 is a cross sectional view of the dual compliant beam electrode actuator-based shutter assembly of FIG. 8;

FIG. 12 is an energy diagram illustrating the energy characteristics of various dual compliant electrode based shutter assemblies, according to an illustrative embodiment of the invention;

FIG. 13A is a top view of a bi-stable dual compliant beam electrode actuator based-shutter assembly, according to an illustrative embodiment of the invention;

FIG. 13B shows the evolution of force versus displacement for a bi-stable shutter assembly;

FIG. 14 is a top view of a second bi-stable dual compliant beam electrode actuator based-shutter assembly, according to an illustrative embodiment of the invention;

FIG. 15 is a top view of a tri-stable shutter assembly incorporating dual compliant electrode actuators, according to an illustrative embodiment of the invention;

FIGS. 16A-C are conceptual diagrams of another embodiment of a bi-stable shutter assembly, illustrating the state of the shutter assembly during a change in shutter position, according to an illustrative embodiment of the invention;

FIG. 17A is a conceptual diagram of a bi-stable shutter assembly including substantially rigid beams, according to an illustrative embodiment of the invention;

FIG. 17B is a top view of a rotational bi-stable shutter assembly;

FIG. 18 is a conceptual diagram of a bi-stable shutter assembly incorporating thermoelectric actuators, according to an illustrative embodiment of the invention;

FIG. 19 is a conceptual diagram of a passive matrix array for controlling bi-stable shutter assemblies, according to an illustrative embodiment of the invention;

FIGS. 20A and 20B are conceptual tiling diagrams for arranging shutter assemblies in a display apparatus;

FIG. 21 is cross-sectional view of a display apparatus, according to an illustrative embodiment of the invention;

FIGS. 22A and 22B are top views of the shutter assembly of FIG. 8 in open and closed states, respectively, according to an illustrative embodiment of the invention;

FIGS. 23A-23D are cross sectional views of shutter assemblies having shutters, which, when in a closed position, overlap apertures formed in an adjacent reflective surface, according to an illustrative embodiment of the invention;

FIG. 24 is a cross sectional view of a first electrowetting-based light modulation array, according to an illustrative embodiment of the invention;

FIG. 25 is a cross sectional view of a second electrowetting-based light modulation array, according to an illustrative embodiment of the invention; and

FIG. 26 is a cross sectional view of a third electrowetting-based light modulation array, according to an illustrative embodiment of the invention.

FIGS. 27A and 27B are cross sectional views of an aperture plate, according to an illustrative embodiment of the invention.

FIG. 28 is a cross sectional view of a display assembly, according to an illustrative embodiment of the invention.

FIG. 29A is a cross sectional view of a display assembly, according to an illustrative embodiment of the invention.

FIG. 29B is a cross sectional view of a display assembly, according to an illustrative embodiment of the invention.

FIGS. 30A and 30B are perspective views of a shutter assembly in open and closed states, respectively, according to an illustrative embodiment of the invention.

FIG. 31 is a cross sectional view of a display assembly, according to an illustrative embodiment of the invention.

FIG. 32 is a cross sectional view of a display assembly, according to an illustrative embodiment of the invention.

DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

FIG. 1 is an isometric view of a display apparatus 100, according to an illustrative embodiment of the invention. The display apparatus 100 includes a plurality of light modulators, in particular, a plurality of shutter assemblies 102 a-102 d (generally “shutter assemblies 102”) arranged in rows and columns. In general, a shutter assembly 102 has two states, open and closed (although partial openings can be employed to impart grey scale). Shutter assemblies 102 a and 102 d are in the open state, allowing light to pass. Shutter assemblies 102 b and 102 c are in the closed state, obstructing the passage of light. By selectively setting the states of the shutter assemblies 102 a-102 d, the display apparatus 100 can be utilized to form an image 104 for a projection or backlit display, if illuminated by lamp 105. In another implementation the apparatus 100 may form an image by reflection of ambient light originating from the front of the apparatus. In the display apparatus 100, each shutter assembly 102 corresponds to a pixel 106 in the image 104.

Each shutter assembly 102 includes a shutter 112 and an aperture 114. To illuminate a pixel 106 in the image 104, the shutter 112 is positioned such that it allows light to pass, without any significant obstruction, through the aperture 114 towards a viewer. To keep a pixel 106 unlit, the shutter 112 is positioned such that it obstructs the passage of light through the aperture 114. The aperture 114 is defined by an opening patterned through a reflective or light-absorbing material in each shutter assembly 102.

In alternative implementations, a display apparatus 100 includes multiple shutter assemblies 102 for each pixel 106. For example, the display apparatus 100 may include three color-specific shutter assemblies 102. By selectively opening one or more of the color-specific shutter assemblies 102 corresponding to a particular pixel 106, the display apparatus 100 can generate a color pixel 106 in the image 104. In another example, the display apparatus 100 includes two or more shutter assemblies 102 per pixel 106 to provide grayscale in an image 104. In still other implementations, the display apparatus 100 may include other forms of light modulators, such as micromirrors, filters, polarizers, interferometric devices, and other suitable devices, instead of shutter assemblies 102 to modulate light to form an image.

The shutter assemblies 102 of the display apparatus 100 are formed using standard micromachining techniques known in the art, including lithography; etching techniques, such as wet chemical, dry, and photoresist removal; thermal oxidation of silicon; electroplating and electroless plating; diffusion processes, such as boron, phosphorus, arsenic, and antimony diffusion; ion implantation; film deposition, such as evaporation (filament, electron beam, flash, and shadowing and step coverage), sputtering, chemical vapor deposition (CVD), plasma enhanced CVD, epitaxy (vapor phase, liquid phase, and molecular beam), electroplating, screen printing, and lamination. See generally Jaeger, Introduction to Microelectronic Fabrication (Addison-Wesley Publishing Co., Reading Mass. 1988); Runyan, et al., Semiconductor Integrated Circuit Processing Technology (Addison-Wesley Publishing Co., Reading Mass. 1990); Proceedings of the IEEE Micro Electro Mechanical Systems Conference 1987-1998; Rai-Choudhury, ed., Handbook of Microlithography, Micromachining & Microfabrication (SPIE Optical Engineering Press, Bellingham, Wash. 1997).

More specifically, multiple layers of material (typically alternating between metals and dielectrics) are deposited on top of a substrate forming a stack. After one or more layers of material are added to the stack, patterns are applied to a top most layer of the stack marking material either to be removed from, or to remain on, the stack. Various etching techniques, including wet or dry etches or reactive ion etching, are then applied to the patterned stack to remove unwanted material. The etch process may remove material from one or more layers of the stack based on the chemistry of the etch, the layers in the stack, and the amount of time the etch is applied. The manufacturing process may include multiple iterations of layering, patterning, and etching.

In one implementation the shutter assemblies 102 are fabricated upon a transparent glass or plastic substrate. This substrate may be made an integral part of a backlight which acts to evenly distribute the illumination from lamp 105 before the light exits through apertures 114. Alternatively, and optionally, the transparent substrate may be placed on top of a planar light guide, wherein the array of shutter assemblies 102 act as light modulation elements in the formation of an image. In one implementation the shutter assemblies 102 are fabricated in conjunction with or subsequent to the fabrication of a thin film transistor (TFT) array on the same glass or plastic substrate. The TFT array provides a switching matrix for distribution of electrical signals to the shutter assemblies.

The process also includes a release step. To provide freedom for parts to move in the resulting device, sacrificial material is interdisposed in the stack proximate to material that will form moving parts in the completed device. An etch removes much of the sacrificial material, thereby freeing the parts to move.

After release, one or more of the surfaces of the shutter assembly may be insulated so that charge does not transfer between moving parts upon contact. This can be accomplished by thermal oxidation and/or by conformal chemical vapor deposition of an insulator such as Al2O3, Cr2O3, TiO2, TiSiO4, HfO2, HfSiO4, V2O5, Nb2O5, Ta2O5, SiO2, or Si3N4 or by depositing similar materials using techniques such as atomic layer deposition and others. The insulated surfaces are chemically passivated to prevent problems such as stiction between surfaces in contact by chemical conversion processes such as fluoridation, silanization, or hydrogenation of the insulated surfaces.

Dual compliant electrode actuators make up one suitable class of actuators for driving the shutters 112 in the shutter assemblies 102. A dual compliant beam electrode actuator, in general, is formed from two or more at least partially compliant beams. At least two of the beams serve as electrodes (also referred to herein as “beam electrodes”). In response to applying a voltage across the beam electrodes, the beam electrodes are attracted to one another from the resultant electrostatic forces. Both beams in a dual compliant beam electrode are, at least in part, compliant. That is, at least some portion of each of the beams can flex and/or bend to aid in the beams being brought together. In some implementations the compliance is achieved by the inclusion of flexures or pin joints. Some portion of the beams may be substantially rigid or fixed in place. Preferably, at least the majority of the length of the beams are compliant.

Dual compliant electrode actuators have advantages over other actuators known in the art. Electrostatic comb drives are well suited for actuating over relatively long distances, but can generate only relatively weak forces. Parallel plate or parallel beam actuators can generate relatively large forces but require small gaps between the parallel plates or beams and therefore only actuate over relatively small distances. R. Legtenberg et. al. (Journal of Microelectromechanical Systems v. 6, p. 257, 1997) demonstrated how the use of curved electrode actuators can generate relatively large forces and result in relatively large displacements. The voltages required to initiate actuation in Legtenberg, however, are still substantial. As shown herein such voltages can be reduced by allowing for the movement or flexure of both electrodes.

In a dual compliant beam electrode actuator-based shutter assembly, a shutter is coupled to at least one beam of a dual compliant beam electrode actuator. As one of the beams in the actuator is pulled towards the other, the pulled beam moves the shutter, too. In doing so, the shutter is moved from a first position to a second position. In one of the positions, the shutter interacts with light in an optical path by, for example, and without limitation, blocking, reflecting, absorbing, filtering, polarizing, diffracting, or otherwise altering a property or path of the light. The shutter may be coated with a reflective or light absorbing film to improve its interferential properties. In the second position, the shutter allows the light to pass by, relatively unobstructed.

FIGS. 2A and 2B are diagrams of two embodiments of cantilever dual compliant beam electrode actuator based-shutter assemblies for use in a display apparatus, such as display apparatus 100. More particularly, FIG. 2A depicts a cantilever dual compliant beam electrode actuator-based shutter assembly 200 a. The shutter assembly 200 a modulates light to form an image by controllably moving a shutter 202 a in and out of an optical path of light. In one embodiment, the optical path begins behind a surface 204 a, to which the shutter 202 a is attached. The surface 204 a is illustrated as a boundary line. However, the surface 204 a extends beyond the space delimited by the boundary line. Similar boundary lines are used in other figures and may also indicate surfaces which extend beyond the space delimited by the boundary line. The light passes through an aperture 206 a in the surface 204 a towards a viewer or towards a display screen. In another embodiment, the optical path begins in front of the surface 204 a and is reflected back to the viewer from the surface of the aperture 206 a.

The shutter 202 a of the shutter assembly 200 a is formed from a solid, substantially planar, body. The shutter 202 a can take virtually any shape, either regular or irregular, such that in a closed position the shutter 202 a sufficiently obstructs the optical path through the aperture 206 a in the surface 204 a. In addition, the shutter 202 a must have a width consistent with the width of the aperture, that, in the open position (as depicted), sufficient light can pass through the aperture 206 a in the surface 204 a to illuminate a pixel, or contribute to the illumination of a pixel, in the display apparatus.

The shutter 202 a couples to one end of a load beam 208 a. A load anchor 210 a, at the opposite end of the load beam 208 a physically connects the load beam 208 a to the surface 204 a and electrically connects the load beam 208 a to driver circuitry in the surface 204 a. Together, the load 208 a beam and load anchor 210 a serve as a mechanical support for supporting the shutter 202 a over the surface 204 a.

The shutter assembly 200 a includes a pair of drive beams 212 a and 214 a, one located along either side of the load beam 210 a. Together, the drive beams 212 a and 214 a and the load beam 210 a form an actuator. One drive beam 212 a serves as a shutter open electrode and the other drive beam 214 a serves as a shutter close electrode. Drive anchors 216 a and 218 a located at the ends of the drive beams 212 a and 214 a closest to the shutter 202 a physically and electrically connects each drive beam 212 a and 214 a to the surface 204 a. In this embodiment, the other ends and most of the lengths of the drive beams 212 a and 214 a remain unanchored or free. The free ends of the drive beams 212 a and 214 a are closer to the anchored end of the load beam 208 a than the anchored ends of the drive beams 212 a and 214 a are to the shutter end of the load beam 208 a.

The load beam 208 a and the drive beams 212 a and 214 a are compliant. That is, they have sufficient flexibility and resiliency that they can be bent out of their unstressed (“rest”) position or shape to at least some useful degree, without fatigue or fracture. As the load beam 208 a and the drive beams 212 a and 214 a are anchored only at one end, the majority of the lengths of the beams 208 a, 212 a, and 214 a is free to move, bend; flex, or deform in response to an applied force. The operation of the cantilever dual compliant beam electrode actuator based-shutter assembly 200 a is discussed further below in relation to FIG. 3.

FIG. 2B is a second illustrative embodiment of a cantilever dual compliant beam electrode actuator-based shutter assembly 200 b. Like the shutter assembly 200 a, the shutter assembly 200 b includes a shutter 202 b, coupled to a load beam 208 b, and two drive beams 212 b and 214 b. The shutter 202 b is positioned in between its fully open position and its fully closed position. The load beam 208 b and the drive beams 212 b and 214 b, together, form an actuator. Drive anchors 210 b, 216 b and 218 b, coupled to each end of the beams connect the beams to a surface 204 b. In contrast to the shutter assembly 200 a, the shutter of shutter assembly 200 b includes several shutter apertures 220, in the form of slots. The surface 204 b, instead of only having one aperture, includes one surface aperture 206 b corresponding to each shutter aperture 220. In the open position, the shutter apertures 220 substantially align with the apertures 206 b in the surface 204 b, allowing light to pass through the shutter 202 b. In the closed position, the surface apertures 206 b are obstructed by the remainder of the shutter 202 b, thereby preventing the passage of light.

Changing the state of a shutter assembly that includes multiple shutter apertures with a corresponding number of surface apertures requires less shutter movement than changing the state of a shutter assembly incorporating a solid shutter and single surface aperture, while still providing for the same aperture area. Reduced required motion corresponds to lower required actuation voltage. More particularly, a decrease in required motion by ⅓ reduces the necessary actuation voltage of the actuator by a factor of about ⅓. Reduced actuation voltage further corresponds to reduced power consumption. Since the total aperture area for either shutter assembly is about the same, each shutter assembly provides a substantially similar brightness.

In other implementations, the shutter apertures and corresponding surface apertures have shapes other than slots. The apertures may be circular, polygonal or irregular. In alternative implementations, the shutter may include more shutter apertures than there are surface apertures in the shutter assembly. In such implementations, one or more of the shutter apertures may serve as a filter, such as color filter. For example, the shutter assembly may have three shutter apertures for every surface aperture, each shutter aperture including a red, blue, or green colored filter.

FIGS. 3A and 3B are diagrams illustrating the relationship between the displacement at the end of the load beam and the relative voltage needed to move the load beam closer to the drive beam. The displacement that can be achieved at any given voltage depends, at least in part, on the curvature or shape of the drive beam, or more precisely, on how the separation, d, and the bending stress along the drive beam and the load beam varies as a function of position x along the load beam A separation function d(x), shown in FIG. 3A can be generalized to the form of d=ax^(n), where y is the distance between the beams. For example, if n=1, the distance between drive electrode and load electrode increase linearly along the length of the load electrode. If n=2, the distance increases parabolically. In general, assuming a constant voltage, as the distance between the compliant electrodes decreases, the electrostatic force at any point on the beams increases proportional to 1/d. At the same time, however, any deformation of the load beam which might decrease the separation distance may also result in a higher stress state in the beam. Below a minimum threshold voltage a limit of deformation will be reached at which any electrical energy released by a closer approach of the electrodes is exactly balanced by the energy which becomes stored in the deformation energy of the beams.

As indicated in the diagram 3B, for actuators having separation functions in which n is less than or equal to 2, the application of a minimum actuation voltage (V₂) results in a cascading attraction of the load beam to the drive beam without requiring the application of a higher voltage. For such actuators, the incremental increase in electrostatic force on the beams resulting from the load beam getting closer to the drive beam is greater than the incremental increase in stress on the beams needed for further displacement of the beams.

For actuators having separation functions in which x is greater than 2, the application of a particular voltage results in a distinct partial displacement of the load electrode. That is, the incremental increase in electrostatic force on the beams resulting from a particular decrease in separation between the beams, at some point, fails to exceed the incremental deformation force needed to be imparted on the load beam to continue reducing the separation. Thus, for actuators having separation functions having n greater than 2, the application of a first voltage level results in a first corresponding displacement of the load electrode. A higher voltage results in a greater corresponding displacement of the load electrode. How the shapes and relative compliance of thin beam electrodes effects actuation voltage is discussed in more detail in the following references: (R. Legtenberg et. al., Journal of Microelectromechanical Systems v. 6, p. 257 (1997) and J. Li et. al. Transducers '03, The 12^(th) International Conference on Solid State Sensors, Actuators, and Microsystems, p. 480 (2003), each of which is incorporated herein by reference.

Referring back to FIGS. 2A and 2B, a display apparatus incorporating the shutter assemblies 202 a and 202 b actuates, i.e., changes the position of the shutter assemblies 202 a and 202 b, by applying an electric potential, from a controllable voltage source, to one of the drive beams 212 a, 212 b, 214 a, or 214 b via its corresponding drive anchor 216 a, 216 b, 218 a, or 218 b, with the load beam 208 a or 208 b being electrically coupled to ground, resulting in a voltage across the beams 208 a, 208 b, 212 a, 212 b, 214 a, 214 b. The controllable voltage source, such as an active matrix array driver, is electrically coupled to load beam 208 a or 208 b via an active matrix array (see FIGS. 9 and 10 below). The display apparatus may instead apply an electric potential to the load beam 208 a or 208 b via the load anchor 210 a or 210 b of the shutter assembly 202 a or 202 b to increase the voltage. An electrical potential difference between the drive beams and the load beams, regardless of sign or ground potential, will generate an electrostatic force between the beams.

With reference back to FIG. 3, the shutter assembly 200 a of FIG. 2A has a second order separation function (i.e., n=2). Thus, if the voltage or potential difference between the beams 208 a and 212 a or 214 a of the shutter assembly 202 a at their point of least separation exceeds the minimum actuation voltage (V₂) the deformation of the beams 208 a and 212 a or 214 a cascades down the entire lengths of the beams 208 a and 212 a or 214 a, pulling the shutter end of the load beam 208 a towards the anchored end of the drive beam 212 a or 214 a. The motion of the load beam 208 a displaces the shutter 202 a such that it changes its position from either open to closed, or visa versa, depending on to which drive beam 212 a or 214 a the display apparatus applied the potential. To reverse the position change, the display apparatus ceases application of the potential to the energized drive beam 212 a or 214 a. Upon the display apparatus ceasing to apply the potential, energy stored in the form of stress on the deformed load beam 208 a restores the load beam 208 a to its original or rest position. To increase the speed of the restoration and to reduce any oscillation about the rest position of the load beam 208 a, the display apparatus may return the shutter 202 a to its prior position by applying an electric potential to the opposing drive beam 212 a or 214 a.

The shutter assemblies 200 a and 200 b, as well as shutter assemblies 500 (see FIG. 5 below), 600 (see FIG. 6 below), 700 (see FIG. 7 below) and 800 (see FIG. 8 below) have the property of being electrically bi-stable. Generally, this is understood to encompass, although not be limited to, devices wherein the electrical potential V₂ that initiates movement between open and closed states is generally greater than the electrical potential (V₁) required to keep the shutter assembly in a stable state. Once the load beam 208 a and one of the drive beams are in contact, a substantially greater electrical force is to be applied from the opposing drive beam to move or separate the load beam, such electrical force being greater than would be necessary if the load beam 208 a were to begin in a neutral or non-contact position. The bistable devices described herein may employ a passive matrix driving scheme for the operation of an array of shutter assemblies such as 200 a. In a passive matrix driving sequence it is possible to preserve an image by maintaining a stabilization voltage V₁ across all shutter assemblies (except those that are being actively driven to a state change). With no or substantially no electrical power required, maintenance of a potential V₁ between the load beam 208 a and drive beam 212 a or 214 a is sufficient to maintain the shutter assembly in either its open or closed states. In order to effect a switching event the voltage between load beam 208 a and the previously affected drive beam (for instance 212 a) is allowed to return from V₁ to zero while the voltage between the load beam 208 a and the opposing beam (for instance 212 b) is brought up to the switching voltage V₂.

In FIG. 2B, the actuator has a third order separation function (i.e., n=3). Thus applying a particular potential to one of the drive beams 212 b or 214 b results in an incremental displacement of the shutter 202 b. The display apparatus takes advantage of the ability to incrementally displace the shutter 202 b to generate a grayscale image. For example, the application of a first potential to a drive beam 212 or 214 b displaces the shutter 202 b to its illustrated position, partially obstructing light passing through the surface apertures 206 b, but still allowing some light to pass through the shutter 202 b. The application of other potentials results in other shutter 202 b positions, including fully open, fully closed, and other intermediate positions between fully open and fully closed. In such fashion electrically analog drive circuitry may be employed in order to achieve an analog grayscale image.

FIGS. 3C through 3F demonstrate the stages of motion of the load beam 208 a, the shutter close electrode 214 a, and the shutter 202 a of the shutter assembly 200 a of FIG. 2A. The initial separation between the compliant beams 208 a and 214 a fits a second order separation function. FIG. 3C shows the load beam 208 a in a neutral position with no voltage applied. The aperture 206 a is half-covered by the shutter 202 a.

FIG. 3D demonstrates the initial steps of actuation. A small voltage is applied between the load beam 208 a and the shutter close electrode 214 a. The free end of the shutter close electrode 214 a has moved to make contact with the load beam 208 a.

FIG. 3E shows the shutter assembly 200 a at a point of actuation after the shutter 202 a begins to move towards the shutter close electrode 214 a.

FIG. 3F shows the end state of actuation of the shutter assembly 200 a. The voltage has exceeded the threshold for actuation. The shutter assembly 200 a is in the closed position. Contact is made between the load beam 208 a and the shutter close electrode 214 a all along its length.

FIG. 4A is a first cross sectional diagram of dual compliant electrode mirror-based light modulator 400 for inclusion in a display apparatus, such as display apparatus 100, instead of, or in addition to, the shutter assemblies 102. The mirror-based-based light modulator 400 includes a mechanically compliant reflection platform 402. At least a portion of the reflection platform 402 is itself reflective or is coated with or is connected to a reflective material.

The reflection platform 402 may or may not be conductive. In implementations in which the reflection platform 402 is conductive, the reflection platform serves as a load electrode for the mirror-based light modulator 400. The reflection platform 402 is physically supported over, and is electrically coupled to, a substrate 404 via a compliant support member 406. If the reflection platform 402 is formed from a non-conductive material, the reflection platform 402 is coupled to a compliant conductive load beam or other form of compliant load electrode. A compliant support member 406 physically supports the combined reflection platform 402 and electrode over the substrate 404. The support member 406 also provides an electrical connection from the electrode to the substrate 404.

The mirror-based light modulator 400 includes a second compliant electrode 408, which serves a drive electrode 408. The drive electrode 408 is supported between the substrate 404 and the reflection platform 402 by a substantially rigid second support member 410. The second support member 410 also electrically connects the second compliant electrode 408 to a voltage source for driving the mirror-based light modulator 400.

The mirror-based light modulator 400 depicted in FIG. 4A is in rest position in which neither of the electrodes 402 or 408 carry a potential. FIG. 4B depicts the mirror-based light modulator 400 in an activated state. When a potential difference is generated between the drive electrode 408 and the load electrode 402 (be it the reflective platform 402 or an attached load beam), the load electrode 402 is drawn towards the drive electrode 408, thereby bending the compliant support beam 406 and angling the reflective portion of the reflection platform 402 to be at least partially transverse to the substrate 404.

To form an image, light 412 is directed at an array of mirror-based light modulators 400 at a particular angle. Mirror-based light modulators 400 in their rest states reflect the light 412 away from the viewer or the display screen, and mirror-based light modulators in the active state reflect the light 412 towards a viewer or a display screen, or visa versa.

FIG. 5 is a diagram of another cantilever dual compliant beam electrode actuator-based shutter assembly 500. As with the shutter assemblies 200 a and 200 b, the shutter assembly 500 includes a shutter 502 coupled to a compliant load beam 504. The compliant load beam 504 is then physically anchored to a surface 506, and electrically coupled to ground, at its opposite end via a load anchor 508. The shutter assembly 500 includes only one compliant drive beam 510, located substantially alongside the load beam 504. The drive beam 510, in response to being energized with an electric potential from a controllable voltage source draws the shutter 502 from a first position (in which the load beam 504 is substantially unstressed) in a plane substantially parallel to the surface, to a second position in which the load beam 504 is stressed. When the potential is removed, the stored stress in the load beam 504 restores the load beam 504 to its original position.

In addition, in comparison to the shutter assemblies 202 a and 202 b, the load beam 504 has a width which varies along its length. The load beam 504 is wider near its anchor 508 than it is nearer to the shutter 502. In comparison to the shutter assemblies 202 a and 202 b and because of its tailored width, the load beam 504 typically has an overall greater stiffness. Shutter assemblies incorporating stiffer beams typically require higher voltages for actuation, but in return, allow for higher switching rates. For example, the shutter assemblies 202 a and 202 b may be switched up to about 10 kHz, while the stiffer shutter assembly 500 may be switched up to about 100 kHz.

FIG. 6 is a diagram of a shutter assembly 600 incorporating two dual compliant electrode beam actuators 602 (“actuators 602”), according to an illustrative embodiment of the invention. The shutter assembly 600 includes a shutter 604. The shutter 604 may be solid, or it may include one or more shutter apertures as described in relation to FIG. 2B. The shutter 604 couples on one side to the beam actuators 602. Together, the actuators 602 move the shutter transversely over a surface in plane of motion which is substantially parallel to the surface.

Each actuator 602 includes a compliant load member 606 connecting the shutter 604 to a load anchor 608. The compliant load members 606 each include a load beam 610 and an L bracket 612. The load anchors 608 along with the compliant load members 606 serve as mechanical supports, keeping the shutter 604 suspended proximate to the surface. The load anchors 608 physically connect the compliant load members 606 and the shutter 604 to the surface and electrically connect the load beams 610 of the load members 606 to ground. The coupling of the shutter 604 from two positions on one side of the shutter 604 to load anchors 608 in positions on either side of the shutter assembly 600 help reduce twisting motion of the shutter 604 about its central axis 614 during motion.

The L brackets 612 reduce the in-plane stiffness of the load beam. 610. That is, the L brackets 612 reduce the resistance of actuators 602 to movement in a plane parallel to the surface (referred to as “in-plane movement” 615), by relieving axial stresses in the load beam.

Each actuator 602 also includes a compliant drive beam 616 positioned adjacent to each load beam 610. The drive beams 616 couple at one end to a drive beam anchor 618 shared between the drive beams 616. The other end of each drive beam 616 is free to move. Each drive beam 616 is curved such that it is closest to the load beam 610 near the free end of the drive beam 616 and the anchored end of the load beam 610.

In operation, a display apparatus incorporating the shutter assembly 600 applies an electric potential to the drive beams 616 via the drive beam anchor 618. As a result of a potential difference between the drive beams 616 and the load beam 610, the free ends of the drive beams 616 are pulled towards the anchored ends of the load beams 610 and the shutter ends of the load beams 610 are pulled toward the anchored ends of the drive beams 616. The electrostatic force draws the shutter 604 towards the drive anchor 618. The compliant members 606 act as springs, such that when the electrical potentials are removed from the drive beams 616, the load beams compliant members 606 push the shutter 604 back into its initial position, releasing the stress stored in the load beams 610. The L brackets 612 also serve as springs, applying further restoration force to the shutter 604.

In fabrication of shutter assemblies 200 through 800, as well as for shutter assemblies 1300 through 1800, it is preferable to provide a rectangular shape for the cross section of the load beams (such as load beams 610) and the drive beams (such as drive beams 616). By providing a beam thickness (in the direction perpendicular to surface) which is 1.4 times or more larger in dimension than the beam width (in a direction parallel to the surface) the stiffness of the load beam 610 will be increased for out-of-plane motion 617 versus in-plane motion 615. Such a dimensional and, by consequence, stiffness differential helps to ensure that the motion of the shutter 604, initiated by the actuators 602, is restricted to motion along the surface and across the surface apertures as opposed to out-of-plane motion 617 which would a wasteful application of energy. It is preferable for certain applications that the cross section of the load beams (such as 610) be rectangular as opposed to curved or elliptical in shape. The strongest actuation force is achieved if the opposing beam electrodes have flat faces so that upon actuation they can approach and touch each other with the smallest possible separation distance.

FIG. 7 is a diagram of a second shutter assembly 700 incorporating two dual compliant electrode beam actuators 702, according to an illustrative embodiment of the invention. The shutter assembly 700 takes the same general form of the shutter assembly 600, other than it includes a return spring 704. As with the shutter assembly 600, in the shutter assembly 700, two actuators 702 couple to a first side of a shutter 706 to translate the shutter 706 in a plane parallel to a surface over which the shutter is physically supported. The return spring 704 couples to the opposite side of the shutter 706. The return spring 704 also couples to the surface at a spring anchor 708, acting as an additional mechanical support. By physically supporting the shutter 706 over the surface at opposite sides of the shutter 706, the actuators 702 and the return spring 704 reduce motion of the shutter 706 out of the plane of intended motion during operation. In addition, the return spring 704 incorporates several bends which reduce the in-plane stiffness of the return spring 704, thereby further promoting in-plane motion over out-of-plane motion. The return spring 704 provides an additional restoration force to the shutter 706, such that once an actuation potential is removed, the shutter 706 returns to its initial position quicker. The addition of the return spring 704 increases only slightly the potential needed to initiate actuation of the actuators 702.

FIG. 8 is a diagram of a shutter assembly 800 including a pair of shutter open actuators 802 and 804 and a pair of shutter close actuators 806 and 808, according to an illustrative embodiment of the invention. Each of the four actuators 802, 804, 806, and 808 take the form of a dual compliant beam electrode actuator. Each actuator 802, 804, 806, and 808 includes a compliant load member 810 coupling a shutter 812, at one end, to a load anchor 814, at the other end. Each compliant load member 810 includes a load beam 816 and an L bracket 818. Each actuator 802, 804, 806, and 808 also includes a drive beam 820 with one end coupled to a drive anchor 822. Each pair of actuators 802/804 and 806/808 share a common drive anchor 822. The unanchored end of each drive beam 820 is positioned proximate to the anchored end of a corresponding compliant load member 810. The anchored end of each drive beam 820 is located proximate to the L bracket end of a corresponding load beam 816. In a deactivated state, the distance between a load beam 816 and its corresponding drive beam 820 increases progressively from the anchored end of the load beam 816 to the L bracket 818.

In operation, to open the shutter 812, a display apparatus incorporating the shutter assembly 800 applies an electric potential to the drive anchor 822 of the shutter open actuators 802 and 804, drawing the shutter 812 towards the open position. To close the shutter 812, the display apparatus applies an electric potential to the drive anchor 822 of the shutter close actuators 806 and 808 drawing the shutter 812 towards the closed position. If neither pair of actuators 802/804 or 806/808 are activated, the shutter 812 remains in an intermediate position, somewhere between fully open and fully closed.

The shutter open actuators 802/804 and shutter closed actuators 806/808 couple to the shutter 812 at opposite ends of the shutter. The shutter open and closed actuators have their own load members 810, thus reducing the actuation voltage of each actuator 802, 804, 806 and 808. Because of the electrical bi-stability described in reference to FIG. 3, it is advantageous to find an actuation method or structure with more leverage for separating the compliant load member 810 from a drive beam 820 with which it might be in contact. By positioning the open and closed actuators 802/804 and 806/808 on opposite sides of the shutter 812, the actuation force of the actuator-to-be-actuated is transferred to the actuator-to-be-separated through the shutter. The actuation force is therefore applied to the task of separation at a point close to the shutter (for instance near the L-bracket end of the load beam 816) where its leverage will be higher.

For shutter assemblies such as in FIG. 8 typical shutter widths (along the direction of the slots) will be in the range of 20 to 800 microns. The “throw distance” or distance over which the shutter will move between open and closed positions will be in the range of 4 to 100 microns. The width of the drive beams and load beams will be in the range of 0.2 to 40 microns. The length of the drive beams and load beams will be in the range of 10 to 600 microns. Such shutter assemblies may be employed for displays with resolutions in the range of 30 to 1000 dots per inch.

Each of the shutter assemblies 200 a, 200 b, 500, 600, 700 and 800, and the mirror-based light modulator 400, described above fall into a class of light modulators referred to herein as “elastic light modulators.” Elastic light modulators have one mechanically stable rest state. In the rest state, the light modulator may be on (open or reflecting), off (closed or not reflecting), or somewhere in between (partially open or partially reflecting). If the generation of a voltage across beams in an actuator forces the light modulator out of its rest state into a mechanically unstable state, some level of voltage across the beams must be maintained for the light modulator to remain in that unstable state.

FIG. 9 is a diagram of an active matrix array 900 for controlling elastic light modulators 902 in a display apparatus. In particular, the active matrix array 900 is suitable for controlling elastic light modulators 902, such as the mirror-based light modulator 400 or shutter-based light modulators 500, 600, and 700, that include only a passive restoration force. That is, these light modulators 902 require electrical activation of actuators to enter a mechanically unstable state, but then utilize mechanical mechanisms, such as springs, to return to the rest state.

The active matrix array is fabricated as a diffused or thin-film-deposited electrical circuit on the surface of a substrate on which the elastic light modulators 902 are formed. The active matrix array 900 includes a series of row electrodes 904 and column electrodes 906 forming a grid like pattern on the substrate, dividing the substrate into a plurality of grid segments 908. The active matrix array 900 includes a set of drivers 910 and an array of non-linear electrical components, comprised of either diodes or transistors that selectively apply potentials to grid segments 908 to control one or more elastic light modulators 902 contained within the grid segments 908. The art of thin film transistor arrays is described in Active Matrix Liquid Crystal Displays: Fundamentals and Applications by Willem den Boer (Elsevier, Amsterdam, 2005).

Each grid segment 908 contributes to the illumination of a pixel, and includes one or more elastic light modulators 902. In grid segments 908, including only a single elastic light modulator 902, the grid segment 908 includes, in addition to the elastic light modulator 902, at least one diode or transistor 912 and optionally a capacitor 914. The capacitor 914 shown in FIG. 9 can be explicitly added as a design element of the circuit, or it can be understood that the capacitor 914 represents the equivalent parallel or parasitic capacitance of the elastic light modulator. The emitter 916 of the transistor 912 is electrically coupled, to either the drive electrode or the load electrode of the elastic light modulator 902. The other electrode of the actuator is coupled to a ground or common potential. The base 918 of the transistor 912 electrically couples to a row electrode 904 controlling a row of grid segments. When the base 918 of the transistor receives a potential via the row electrode 904, current can run through the transistor 912 from a corresponding column electrode 906 to generate a potential in the capacitor 914 and to apply a potential to the drive electrode of the elastic light modulator 902 activating the actuator.

The active matrix array 900 generates an image, in one implementation by, one at a time, applying a potential from one of the drivers 910 to a selected row electrode 904, activating a corresponding row of grid segments 908. While a particular row is activated, the display apparatus applies a potential to the column electrodes corresponding to grid segments in the active row containing light modulators which need to be switched out of a rest state.

When a row is subsequently deactivated, a stored charge will remain on the electrodes of the actuator 902 (as determined by the equivalent capacitance of the actuator) as well as, optionally, on the parallel capacitor 914 that can be designed into the circuit, keeping the elastic shutter mechanisms 902 in their mechanically unstable states. The elastic shutter mechanism 902 remains in the mechanically unstable state until the voltage stored in the capacitor 914 dissipates or until the voltage is intentionally reset to ground potential during a subsequent row selection or activation step.

FIG. 10 is diagram of another implementation of an active matrix array 1000 for controlling elastic light modulators 1002 in a display apparatus. In particular, the active matrix array 1000 is suitable for controlling elastic light modulators, such as shutter-based light modulators 200 a, 200 b, and 800, which include one set of actuators for forcing the light modulators from a rest state to a mechanically unstable state and a second set of actuators for driving the light modulators back to the rest state and possibly to a second mechanically unstable state. Active matrix array 1000 can also be used for driving non-elastic light modulators described further in relation to FIGS. 12-20.

The active matrix array 1000 includes one row electrode 1004 for each row in the active matrix array 1000 and two column electrodes 1006 a and 1006 b for each column in the active matrix array 1000. For example, for display apparatus including shutter-based light modulators, one column electrode 1006 a for each column corresponds to the shutter open actuators of light modulators 1002 in the column. The other column electrode 1006 b corresponds to the shutter close actuators of the light modulators 1002 in the column. The active matrix array 1000 divides the substrate upon which it is deposited into grid sections 1008. Each grid section 1008 includes one or more light modulators 1002 and at least two diodes or transistors 1010 a and 1010 b and optionally two capacitors 1012 a and 1012 b. The bases 1014 a and 1014 b of each transistor 1010 a and 1010 b are electrically coupled to a column electrode 1006 a or 1006 b. The emitters 1016 a and 1016 b of the transistors 1010 a and 1010 b are coupled to a corresponding capacitor 1012 a or 1012 b and a drive electrode of the light modulator(s) 1002 in the grid section 1008.

In operation, a driver applies a potential to a selected row electrode 1004, activating the row. The active matrix array 1000 selectively applies potentials to one of the two column electrodes 1006 a or 1006 b of each column in which the state of the light modulator(s) 1002 in the grid section 1008 needs to be changed. Alternatively, the active matrix array 1000 may also apply a potential to column electrodes 1006 a or 1006 b for grid sections 1008 previously in an active state which are to remain in an active state.

For both active matrix arrays 900 and 1000, the drivers powering the column electrodes, in some implementations, select from multiple possible potentials to apply to individual column electrodes 1006 a and 1006 b. The light modulator(s) 1002 in those columns can then be opened or closed different amounts to create grayscale images.

FIG. 11 is a cross sectional view of the shutter-assembly 800 of FIG. 8 along the line labeled A-A′. Referring to FIGS. 8, 10, and 11, the shutter assembly 800 is built on substrate 1102 which is shared with other shutter assemblies of a display apparatus, such as display apparatus 100, incorporating the shutter assembly 800. The voltage signals to actuate the shutter assembly, are transmitted along conductors in underlying layers of the shutter assembly. is The voltage signals are controlled by an active matrix array, such as active matrix array 1000. The substrate 1102 may support as many as 4,000,000 shutter assemblies, arranged in up to about 2000 rows and up to about 2000 columns.

In addition to the shutter 812, the shutter open actuators 802 and 804, the shutter close actuators 806 and 808, the load anchors 814 and the drive anchors 822, the shutter assembly 800 includes a row electrode 1104, a shutter open electrode 1106, a shutter close electrode 1108, and three surface apertures 1110. The depicted shutter assembly has at least three functional layers, which may be referred to as the row conductor layer, the column conductor layer, and the shutter layer. The shutter assembly is preferably made on a transparent substrate such as glass or plastic. Alternatively the substrate can be made from an opaque material such as silicon, as long as through holes are provided at the positions of each of the surface apertures 1110 for the transmission of light. The first metal layer on top of the substrate is the row conductor layer which is patterned into row conductor electrodes 1104 as well as reflective surface sections 1105. The reflective surface sections 1105 reflect light passing through the substrate 1102 back through the substrate 1102 except at the surface apertures 1110. In some implementations the surface apertures may include or be covered by red, green, or blue color filtering materials.

The shutter open electrode 1106 and the shutter close electrode 1108 are formed in a column conductor layer 1112 deposited on the substrate 1102, on top of the row conductor layer 1104. The column conductor layer 1112 is separated from the row conductor layer 1104 by one or more intervening layers of dielectric material or metal. The shutter open electrode 1104 and the shutter close electrode 1106 of the shutter assembly 800 are shared with other shutter assemblies in the same column of the display apparatus. The column conductor layer 1112 also serves to reflect light passing through gaps in the ground electrode 1104 other than through the surface apertures 1110. The row conductor layer 1104 and the column conductor layer 1112 are between about 0.1 and about 2 microns thick. In alternative implementations, the column conductor 1112 layer can be located below the row conductor layer 1104. In another alternative implementation both the column conductor layer and the row conductor layer may be located above the shutter layer.

The shutter 812, the shutter open actuators 802 and 804, the shutter close actuators 806 and 808, the load anchors 814 and the drive anchors 822 are formed from a third functional layer of the shutter assembly 800, referred to as the shutter layer 1114. The actuators 802, 804, 806, and 808 are formed from a deposited metal, such as, without limitation, Au, Cr or Ni, or a deposited semiconductor, such as, without limitation, polycrystalline silicon, or amorphous silicon, or from single crystal silicon if formed on top of a buried oxide (also known as silicon on insulator). The beams of the actuators 802, 804, 806, and 808 are patterned to dimensions of about 0.2 to about 20 microns in width. The shutter thickness is typically in the range of 0.5 microns to 10 microns. To promote the in-plane movement of the shutters (i.e. reduce the transverse beam stiffness as opposed to the out-of-plane stiffness), it is preferable to maintain a beam dimensional ratio of about at least 1.4:1, with the beams being thicker than they are wide.

Metal or semiconductor vias electrically connect the row electrode 1104 and the shutter open electrode 1106 and the shutter close electrode 1108 of the column conductor layer 1112 to features on the shutter layer 1114. Specifically, vias 1116 electrically couple the row electrode 1104 to the load anchors 814 of the shutter assembly 800, keeping the compliant load member 810 of the shutter open actuators 802 and 804 and the shutter close actuators 806 and 808, as well as the shutter 812, at the row conductor potential. Additional vias electrically couple the shutter open electrode 1106 to the drive beams 820 of the shutter open actuators 802 and 804 via the drive anchor 822 shared by the shutter open actuators 802 and 804. Still other vias electrically couple the shutter close electrode 1108 to the drive beams 820 of the of the shutter close actuators 806 and 808 via the drive anchor 822 shared by the shutter close actuators 806 and 808.

The shutter layer 1114 is separated from the column conductor layer 1112 by a lubricant, vacuum or air, providing the shutter 812 freedom of movement. The moving pieces in the shutter layer 1114 are mechanically separated from neighboring components (except their anchor points 814) in a release step, which can be a chemical etch or ashing process, which removes a sacrificial material from between all moving parts.

The diodes, transistors, and/or capacitors (not shown for purpose of clarity) employed in the active matrix array may be patterned into the existing structure of the three functional layers, or they can be built into separate layers that are disposed either between the shutter assembly and the substrate or on top of the shutter layer. The reflective surface sections 1105 may be patterned as extensions of the row and column conductor electrodes or they can be patterned as free-standing or electrically floating sections of reflective material. Alternatively the reflective surface sections 1105 along with their associated surface apertures 1110 can be patterned into a fourth functional layer, disposed between the shutter assembly and the substrate, and formed from either a deposited metal layer or a dielectric mirror. Grounding conductors may be added separately from the row conductor electrodes in layer 1104. These separate grounding conductors may be required when the rows are activated through transistors, such as is the case with an active matrix array. The grounding conductors can be either laid out in parallel with the row electrodes (and bussed together in the drive circuits), or the grounding electrodes can be placed into separate layers between the shutter assembly and the substrate.

In addition to elastic light modulators, display apparatus can include bi-stable light modulators, for example bi-stable shutter assemblies. As described above, a shutter in an elastic shutter assembly has one mechanically stable position (the “rest position”), with all other shutter positions being mechanically unstable. The shutter of a bi-stable shutter assembly, on the other hand, has two mechanically stable positions, for example, open and closed. Mechanically bi-stable shutter assemblies have the advantage that no voltage is required to maintain the shutters in either the open or the closed positions. Bi-stable shutter assemblies can be further subdivided into two classes: shutter assemblies in which each stable position is substantially energetically equal, and shutter assemblies in which one stable position is energetically preferential to the other mechanically stable position.

FIG. 12 is a diagram 1200 of potential energy stored in three types of shutter assemblies in relation to shutter position. The solid line 1202 corresponds to an elastic shutter assembly. The first dashed line 1204 corresponds to a bi-stable shutter assembly with equal energy stable states. The second dashed line 1206 corresponds to a bi-stable shutter assembly with non-equal energy stable states. As indicated in the energy diagram 1200, the energy curves 1204 and 1206 for the two types of bi-stable shutter assemblies each include two local minima 1208, corresponding to stable shutter positions, such as fully open 1210 and fully closed 1211. As illustrated, energy must be added to the assembly in order to move its shutters out of the positions corresponding to one of the local minima. For the bi-stable shutter assemblies with non-equal-energy mechanically stable shutter positions, however, the work needed to open a shutter 1212 is greater than the work required to close the shutter 1214. For the elastic shutter assembly, on the other hand, opening the shutter requires work 1218, but the shutter closes spontaneously after removal of the control voltage.

FIG. 13A is a top view of a shutter layer 1300 of a bi-stable shutter assembly. The shutter layer 1300 includes a shutter 1302 driven by two dual compliant electrode actuators 1304 and 1306. The shutter 1302 includes three slotted shutter apertures 1308. One dual compliant electrode actuator 1304 serves as a shutter open actuator. The other dual compliant electrode actuator 1306 serves as a shutter close actuator.

Each dual compliant electrode actuator 1304 and 1306 includes a compliant member 1310 connecting the shutter 1302, at about its linear axis 1312, to two load anchors 1314, located in the corners of the shutter layer 1300. The compliant members 1310 each include a conductive load beam 1316, which may have an insulator disposed on part of, or the entirety of its surface. The load beams 1316 serve as mechanical supports, physically supporting the shutter 1302 over a substrate on which the shutter assembly is built. The actuators 1304 and 1306 also each include two compliant drive beams 1318 extending from a shared drive anchor 1320. Each drive anchor 1320 physically and electrically connects the drive beams 1318 to the substrate. The drive beams 1318 of the actuators 1304 and 1306 curve away from their corresponding drive anchors 1320 towards the points on the load anchors 1314 at which load beams 1316 couple to the load anchors 1314. These curves in the drive beams 1318 act to reduce the stiffness of the drive beams, thereby helping to decrease the actuation voltage.

Each load beam 1316 is generally curved, for example in a bowed (or sinusoidal) shape. The extent of the bow is determined by the relative distance between the load anchors 1314 and the length of the load beam 1316. The curvatures of the load beams 1316 provide the bi-stability for the shutter assembly 1300. As the load beam 1316 is compliant, the load beam 1316 can either bow towards or away from the drive anchor 1320. The direction of the bow changes depending on what position the shutter 1302 is in. As depicted, the shutter 1302 is in the closed position. The load beam 1316 of the shutter open actuator 1304 bows away from the drive anchor 1320 of the shutter open actuator 1304. The load beam 1316 of the shutter closed actuator 1306 bows towards the drive anchor 1320 of the shutter close actuator 1306.

In operation, to change states, for example from closed to open, a display apparatus applies a potential to the drive beams 1318 of the shutter open actuator 1304. The display apparatus may also apply a potential to the load beams 1316 of the shutter open actuator. Any electrical potential difference between the drive beams and the load beams, regardless of sign with respect to a ground potential, will generate an electrostatic force between the beams. The resultant voltage between the drive beams 1318 and the load beams 1316 of the shutter open actuator 1304 results in an electrostatic force, drawing the beams 1316 and 1318 together. If the voltage is sufficiently strong, the load beam 1316 deforms until its curvature is substantially reversed, as depicted in the shutter close actuator in FIG. 13A.

FIG. 13B shows the evolution of force versus displacement for the general case of bi-stable actuation, including that for FIG. 13A. Referring to FIGS. 13A and 13B, generally the force required to deform a compliant load beam will increase with the amount of displacement. However, in the case of a bi-stable mechanism, such as illustrated in FIG. 13A, a point is reached (point B in FIG. 13B) where further travel leads to a decrease in force. With sufficient voltage applied between the load beam 1316 and the drive beam 1318 of the shutter open actuator 1304, a deformation corresponding to point B of FIG. 13B is reached, where further application of force leads to a large and spontaneous deformation (a “snap through”) and the deformation comes to rest at point C in FIG. 13B. Upon removal of a voltage, the mechanism will relax to a point of stability, or zero force. Point D is such a relaxation or stable point representing the open position. To move the shutter 1302 in the opposite direction it is first necessary to apply a voltage between the load beam 1316 and the drive beam 1318 of the shutter close actuator 1306. Again a point is reached where further forcing results in a large and spontaneous deformation (point E). Further forcing in the closed direction results in a deformation represented by point F. Upon removal of the voltage, the mechanism relaxes to its initial and stable closed position, point A.

In FIG. 13A, the length of the compliant member is longer than the straight-line distance between the anchor and the attachment point at the shutter. Constrained by the anchor points, the load beam finds a stable shape by adapting a curved shape, two of which shapes constitute configurations of local minima in the potential energy. Other configurations of the load beam involve deformations with additional strain energy.

For load beams fabricated in silicon, typical as-designed widths are about 0.2 μm to about 10 μm. Typical as-designed lengths are about 20 μm to about 1000 μm. Typical as-designed beam thicknesses are about 0.2 μm to about 10 μm. The amount by which the load beam is pre-bent is typically greater than three times the as-designed width

The load beams of FIG. 13A can be designed such that one of the two curved positions is close to a global minimum, i.e. possesses the lowest energy or relaxed state, typically a state close to zero energy stored as a deformation or stress in the beam. Such a design configuration may be referred to as “pre-bent,” meaning, among other things, that the shape of the compliant member is patterned into the mask such that little or no deformation is required after release of the shutter assembly from the substrate. The as-designed and curved shape of the compliant member is close to its stable or relaxed state. Such a relaxed state holds for one of the two shutter positions, either the open or the closed position. When switching the shutter assembly into the other stable state (which can be referred to as a metastable state) some strain energy will have to be stored in the deformation of the beam; the two states will therefore have unequal potential energies; and less electrical energy will be required to move the beam from metastable to stable states as compared to the motion from the stable state to the metastable state.

Another design configuration for FIG. 13A, however, can be described as a pre-stressed design. The pre-stressed design provides for two stable states with equivalent potential energies. This can be achieved for instance by patterning the compliant member such that upon release of the shutter assembly, the compliant member will substantially and spontaneously deform into its stable shape (i.e. the initial state is designed to be unstable). Preferably the two stable shapes are similar such that the deformation or strain energy stored in the compliant member of each of those stable states will be similar. The work required to move between open and closed shutter positions for a pre-stressed design will be similar.

The pre-stress condition of the shutter assembly can be provided by a number of means. The condition can be imposed post-manufacture by, for instance, mechanically packaging the substrate to induce a substrate curvature and thus a surface strain in the system. A pre-stressed condition can also be imposed as a thin film stress imposed by surface layers on or around the load beams. These thin film stresses result from the particulars of a deposition process. Deposition parameters that can impart a thin film stress include thin film material composition, deposition rate, and ion bombardment rate during the deposition process.

In FIG. 13A, the load beam is curved in each of its locally stable states and the load beam is also curved at all points of deformation in between the stable states. The compliant member may be comprised, however, of any number of straight or rigid sections of load beam as will be described in the following figures. In FIG. 18, furthermore, will be shown the design of a bi-stable shutter assembly in which neither of the two equivalent stable positions possesses, requires, or accumulates any significant deformation or strain energy. Stress is stored in the system temporarily as it is moved between the stable states.

FIG. 14 is a top view of the shutter layer 1400 of a second bi-stable shutter assembly. As described above in relation to FIG. 6, reducing resistance to in-plane motion tends to reduce out-of-plane movement of the shutter. The shutter layer 1400 is similar to that of the shutter layer 1300, other than the shutter layer 1400 includes an in-plane stiffness-reducing feature, which promotes in-plane movement, and a deformation promoter which promotes proper transition between states. As with the shutter layer 1300 of FIG. 13A, the shutter layer 1400 of FIG. 14 includes load beams 1402 coupling load anchors 1404 to a shutter 1406. To reduce the in-plane stiffness of the shutter assembly and to provide some axial compliance to the load beams 1402, the load anchors 1404 couple to the load beams 1402 via springs 1408. The springs 1408 can be formed from flexures, L brackets, or curved portions of the load beams 1402.

In addition, the widths of the load beams 1402 vary along their lengths. In particular, the beams are narrower along sections where they meet the load anchors 1404 and the shutter 1406. The points along the load beams 1402 at which the load beams 1402 become wider serve as pivot points 1410 to confine deformation of the load beams 1402 to the narrower sections 1410.

FIG. 15 is a top view of a shutter layer 1500 of a tri-stable shutter assembly incorporating dual compliant electrode actuators, according to an illustrative embodiment of the invention. The shutter layer 1500 includes a shutter open actuator 1502 and a shutter close actuator 1504. Each actuator 1502 and 1504 includes two compliant drive beams 1506 physically and electrically coupled to a substrate of a display apparatus by a drive anchor 1508.

The shutter open actuator 1502, by itself, is an elastic actuator, having one mechanically stable state. Unless otherwise constrained, the shutter open actuator 1502, after actuation would return to its rest state. The shutter open actuator 1502 includes two load beams 1510 coupled to load anchors 1512 by L brackets 1514 at one end and to the shutter 1516 via L brackets 1518 at the other end. In the rest state of the shutter open actuator 1502, the load beams 1510 are straight. The L brackets 1514 and 1518 allow the load beams 1510 to deform towards the drive beams 1506 of the shutter open actuator 1502 upon actuation of the shutter open actuator 1502 and away from the drive beams 1506 upon actuation of the shutter close actuator 1504.

The shutter close actuator 1504 is similarly inherently elastic. The shutter close actuator 1504 includes a single load beam 1520 coupled to a load anchor 1522 at one end. When not under stress, i.e., in its rest state, the load beam 1520 is straight. At the opposite end of the load beam 1520 of the shutter close actuator 1504, the load beam 1520 is coupled to a stabilizer 1524 formed from two curved compliant beams 1526 connected at their ends and at the center of their lengths. The beams 1526 of the stabilizer 1524 have two mechanically stable positions: bowed away from the shutter close actuator 1504 (as depicted) and bowed towards the shutter close actuator 1504.

In operation, if either the shutter open actuator 1502 or the shutter close actuator are activated 1504, the load beam 1520 of the shutter close actuator 1504 is deformed to bow towards the shutter open actuator 1504 or towards the drive beams 1528 of the shutter close actuator 1504, respectively, as the shutter 1516 is moved into an actuated position. In either case, the length of the shutter close actuator 1504 load beam 1520 with respect to the width of the shutter layer 1500 as a whole, is reduced, pulling the beams 1526 of the stabilizer 1524 to bow towards the shutter close actuator 1504. After the activated actuator is deactivated, the energy needed to deform the beams 1526 of the stabilizer 1524 back to its original position is greater than the energy stored in the load beams 1510 and 1520 and of the actuators 1502 and 1504. Additional energy must be added to the system to return the shutter 1516 to its rest position. Thus, the shutter 1516 in the shutter assembly has three mechanically stable positions, open, half open, and closed.

FIGS. 16A-C are diagrams of another embodiment of a bi-stable shutter assembly 1600, illustrating the state of the shutter assembly 1600 during a change in shutter 1602 position. The shutter assembly 1600 includes a shutter 1602 physically supported by a pair of compliant support beams 1604. The support beams couple to anchors 1603 as well as to the shutter 1602 by means of rotary joints 1605. These joints may be understood to consist of pin joints, flexures or thin connector beams. In the absence of stress being applied to the support beams 1604, the support beams 1604 are substantially straight.

FIG. 16A depicts the shutter 1602 in an open position, FIG. 16B depicts the shutter 1602 in the midst of a transition to the closed position, and FIG. 16C shows the shutter 1602 in a closed position. The shutter assembly 1600 relies upon an electrostatic comb drive for actuation. The comb drive is comprised of a rigid open electrode 1608 and a rigid closed electrode 1610. The shutter 1602 also adopts a comb shape which is complementary to the shape of the open and closed electrodes. Comb drives such as are shown in FIG. 16 are capable of actuating over reasonably long translational distances, but at a cost of a reduced actuation force. The primary electrical fields between electrodes in a comb drive are aligned generally perpendicular to the direction of travel, therefore the force of actuation is generally not along the lines of the greatest electrical pressure experienced by the interior surfaces of the comb drive.

Unlike the bi-stable shutter assemblies described above, instead of relying upon a particular curvature of one or more beams to provide mechanical stability, the bi-stable actuator 1600 relies on the straight relaxed state of its support beams 1604 to provide mechanical stability. For example, in its two mechanically stable positions, depicted in FIGS. 16A and 16C, the compliant support beams 1604 are substantially straight at an angle to the linear axis 1606 of the shutter assembly 1600. As depicted in FIG. 16B, in which the shutter 1602 is in transition from one mechanically stable position to the other, the support beams 1604 physically deform or buckle to accommodate the movement. The force needed to change the position of the shutter 1602 must therefore be sufficient to overcome the resultant stress on the compliant support beams 1604. Any energy difference between the open and closed states of shutter assembly 1600 is represented by a small amount of elastic energy in the rotary joints 1605.

The shutter 1602 is coupled to two positions on either side of the shutter 1602 through support beams 1604 to anchors 1603 in positions on either side of the shutter assembly 1600, thereby reducing any twisting or rotational motion of the shutter 1602 about its central axis. The use of compliant support beams 1604 connected to separate anchors on opposite sides of the shutter 1602 also constrains the movement of the shutter along a linear translational axis. In another implementation, a pair of substantially parallel compliant support beams 1604 can be coupled to each side of shutter 1602. Each of the four support beams couples at independent and opposing points on the shutter 1602. This parallelogram approach to support of the shutter 1602 helps to guarantee that linear translational motion of the shutter is possible.

FIG. 17A depicts a bi-stable shutter assembly 1700 a, in which the beams 1702 a incorporated into the shutter assembly 1700 a are substantially rigid as opposed to compliant, in both of the shutter assembly's stable positions 17A-1 and 17A-3 as well as in a transitional position 17A-2. The shutter assembly 1700 a includes a shutter 1704 a driven by a pair of dual compliant beam electrode actuators 1706 a. Two compliant members 1710 a support the shutter 1704 a over a surface 1712 a. The compliant members 1710 a couple to opposite sides of the shutter 1704 a. The other ends of the compliant members 1710 a couple to anchors 1714 a, connecting the compliant members 1710 a to the surface 1712 a. Each compliant member 1710 a includes two substantially rigid beams 1716 a coupled to a flexure or other compliant element 1718 a, such as a spring or cantilever arm. Even though the beams 1716 a in the compliant members are rigid, the incorporation of the compliant element 1718 a allows the compliant member 1710 a as a whole to change its shape in a compliant fashion to take on two mechanically stable shapes. The compliant element is allowed to relax to its rest state in either of the closed or open positions of the shutter assembly (see 17A-1 and 17A-3), so that both of the end states possess substantially identical potential energies. No permanent beam bending or beam stressing is required to establish the stability of the two end states, although strain energy is stored in the compliant element 1718 a during the transition between states (see 17A-2).

The shape of the compliant element 1718 a is such that a relatively easy in-plane translation of the shutter 1704 a is allowed while out-of-plane motion of the shutter is restricted.

The actuation of the bi-stable shutter assembly 1700 a is accomplished by a pair of elastic dual compliant beam electrode actuators 1706 a, similar to the actuators employed in FIG. 15. In shutter assembly 1700 a the actuators 1706 a are physically separated and distinct from the compliant members 1710 a. The compliant members 1710 a provide a relatively rigid support for the shutter 1704 a while providing the bi-stability required to sustain the open and closed states. The actuators 1706 a provide the driving force necessary to switch the shutter between the open and closed states.

Each actuator 1706 a comprises a compliant load member 1720 a. One end of the compliant load member 1720 a is coupled to the shutter 1704 a, while the other end is free. In shutter assembly 1700 a the compliant load members in actuators 1706 a are not coupled to anchors or otherwise connected to the surface 1712 a. The drive beams 1722 a of the actuators 1706 a are coupled to anchors 1724 a and thereby connected to the surface 1712 a. In this fashion the voltage of actuation is reduced.

FIG. 17B is a diagram of a bi-stable shutter assembly 1700 b in which the shutter 1702 b is designed to rotate upon actuation. The shutter 1702 b is supported at four points along its periphery by 4 compliant support beams 1704 b which are coupled to four anchors 1706 b. As in FIG. 16, the compliant support beams 1704 b are substantially straight in their rest state. Upon rotation of the shutter 1702 b the compliant members will deform as the distance between the anchors and the shutter periphery decreases. There are two low energy stable states in which the compliant support beams 1704 b are substantially straight. The shutter mechanism in 1700 b has the advantage that there is no center of mass motion in the shutter 1702 b.

The shutter 1702 b in shutter assembly 1700 b has a plurality of shutter apertures 1708 b, each of which possesses a segmented shape designed to make maximum use of the rotational motion of the shutter.

FIG. 18 is a diagram of a bi-stable shutter assembly 1800 incorporating thermoelectric actuators 1802 and 1804. The shutter assembly 1800 includes a shutter 1806 with a set of slotted shutter apertures 1808. Thermoelectric actuators 1802 and 1804 couple to either side of the shutter 1806 for moving the shutter 1806 transversely in a plane substantially parallel to a surface 1808 over which the shutter 1806 is supported. The coupling of the shutter 1806 from two positions on either side of the shutter 1806 to load anchors 1807 in positions on either side of the shutter assembly 1800 help reduce any twisting or rotational motion of the shutter 1806 about its central axis.

Each thermoelectric actuator 1802 and 1804 includes three compliant beams 1810, 1812, and 1814. Compliant beams 1810 and 1812 are each thinner than compliant beam 1814. Each of the beams 1810, 1812, and 1814 is curved in an s-like shape, holding the shutter 1806 stably in position.

In operation, to change the position of the shutter from open (as depicted) to closed, current is passed through a circuit including beams 1810 and 1814. The thinner beams 1810 in each actuator 1802 and 1804 heat, and therefore also expands, faster than the thicker beam 1814. The expansion forces the beams 1810, 1812, and 1814 from their mechanically stable curvature, resulting in transverse motion of the shutter 1806 to the closed position. To open the shutter 1806, current is run through a circuit including beams 1812 and 1814, resulting in a similar disproportionate heating and expansion of beams 1812, resulting in the shutter 1806 being forced back to the open position.

Bi-stable shutter assemblies can be driven using a passive matrix array or an active matrix array. FIG. 19 is a diagram of a passive matrix array 1900 for controlling bi-stable shutter assemblies 1902 to generate an image. As with active matrix arrays, such as active matrix arrays 900 and 1000, the passive matrix array 1900 is fabricated as a diffused or thin-film-deposited electrical circuit on a substrate 1904 of a display apparatus. In general, passive matrix arrays 1900 require less circuitry to implement than active matrix arrays 900 and 1000, and are easier to fabricate. The passive matrix array 1900 divides the shutter assemblies 1902 on the substrate 1904 of the display apparatus into rows and columns of grid segments 1906 of a grid. Each grid segment 1906 may include one or more bi-stable shutter assemblies 1902. In the display apparatus, all grid segments 1906 in a given row of the grid share a single row electrode 1908. Each row electrode 1908 electrically couples a controllable voltage source, such as driver 1910 to the load anchors of the shutter assemblies 1902. All shutter assemblies 1902 in a column share two common column electrodes, a shutter open electrode 1912 and a shutter close electrode 1914. The shutter open electrode 1912 for a given column electrically couples a driver 1910 to the drive electrode of the shutter open actuator of the shutter assemblies 1902 in the column. The shutter close electrode 1914 for a given column electrically couples a driver 1910 to the drive electrode of the shutter close actuator of the shutter assemblies 1902 in the column.

The shutter assemblies 1300, 1400, 1500, 1600, 1700 a, and 1800 are amenable to the use of a passive matrix array because their property of mechanical bi-stability makes it possible to switch between open and closed states if the voltage across the actuator exceeds a minimum threshold voltage. If the drivers 1910 are programmed such that none of them will output a voltage that by itself is sufficient to switch the shutter assemblies between open and closed states, then a given shutter assembly will be switched if its actuator receives voltages from two opposing drivers 1910. The shutter assembly at the intersection of a particular row and column can be switched if it receives voltages from its particular row and column drivers whose difference exceeds the minimum threshold voltage.

To change the state of a shutter assembly 1902 from a closed state to an open state, i.e., to open the shutter assembly 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the grid in which the shutter assembly 1902 is located. A second driver 1910 applies a second potential, in some cases having an opposite polarity, to the shutter open electrode 1912 corresponding to the column in the grid in which the shutter assembly 1902 is located. To change the state of a shutter assembly 1902 from an open state to a closed state, i.e., to close the shutter assembly 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the display apparatus in which the shutter assembly 1902 is located. A second driver 1910 applies a second potential, in some cases having an opposite polarity, to the shutter close electrode 1914 corresponding to the column in the display apparatus in which the shutter assembly 1902 is located. In one implementation, a shutter assembly 1902 changes state in response to the difference in potential applied to the row electrode 1908 and one of the column electrodes 1912 or 1914 exceeding a predetermined switching threshold.

To form an image, in one implementation, a display apparatus sets the state of the shutter assemblies 1902 in the grid, one row at a time in sequential order. For a given row, the display apparatus first closes each shutter assembly 1902 in the row by applying a potential to the corresponding row electrodes 1908 and a pulse of potential to all of the shutter close electrodes 1914. Then, the display apparatus opens the shutter assemblies 1902 through which light is to pass by applying a potential to the shutter open electrode 1912 and applying a potential to the row electrodes 1908 for the rows which include shutter assemblies 1902 in the row which are to be opened. In one alternative mode of operation, instead of closing each row of shutter assemblies 1902 sequentially, after all rows in the display apparatus are set to the proper position to form an image, the display apparatus globally resets all shutter assemblies 1902 at the same time by applying potentials to all shutter close electrodes 1914 and all row electrodes 1908 concurrently. In another alternative mode of operation, the display apparatus forgoes resetting the shutter assemblies 1902 and only alters the states of shutter assemblies 1902 that need to change state to display a subsequent image. A number of alternate driver control schemes for images have been proposed for use with ferroelectric liquid crystal displays, many of which can be incorporated for use with the mechanically bi-stable displays herein. These technologies are described in Liquid Crystal Displays: Driving Schemes and Electro-Optical Effects, Ernst Lieder (Wiley, New York, 2001).

The physical layout of the display is often a compromise between the characteristics of resolution, aperture area, and driving voltage. Small pixel sizes are generally sought to increase the resolution of the display. As pixels become smaller, however, proportionally the room available for shutter apertures decreases. Designers seek to maximize aperture ratio as this increases the brightness and power efficiency of the display. Additionally, the combination of a small pixels and large aperture ratios implies large angular deformations in the compliant members that support the shutters, which tends to increase the drive voltages required and the energy dissipated by the switching circuitry.

FIGS. 20A and 20B demonstrate two methods of tiling shutter assemblies into an array of pixels to maximize the aperture ratios in dense arrays and minimize the drive voltages.

FIG. 20A, for example, depicts a tiling 2000 of two cantilever dual beam electrode actuator-based shutter assemblies 2002 and 2004 tiled to form a rhombehedral pixel 2006 from two generally triangular shutter assemblies 2002 and 2004. The shutter assemblies 2002 and 2004 may be independently or collectively controlled. The rhombehedral tiling of FIG. 20A is quite close to a rectangular tiling arrangement, and in fact adapted to a rectangular pixel with aspect ratio of 2:1. Since two shutter assemblies can be established within each rectangle, such a 2:1 rectangular tiling arrangement can further be attached or built on top of an active matrix array which possesses a square repeating distance between rows and columns. A 1 to 1 correlation between pixels in the two arrays can therefore be established. Square pixel arrays are most commonly employed for the display of text and graphic images. The advantage of the layout in FIG. 20B is that it is understood to maximize the length of the load beams in each triangular pixel to reduce the voltage required for switching shutters between open and closed states.

FIG. 20B is an illustrative tiling of a plurality of bi-stable dual compliant beam electrode-actuator-based shutter assemblies 1300 of FIG. 13A. In comparison, for example, to the bi-stable dual compliant beam electrode-actuator-based shutter assembly 1400 of FIG. 14, the width of the shutter 1302 of the shutter assembly 1300 is substantially less than the distance between the load anchors 1314 of the shutter assembly 1300. While the narrower shutter 1302 allows for less light to pass through each shutter assembly 1300, the extra space can be utilized for tighter packing of shutter assemblies 1300, as depicted in FIG. 20B, without loss of length in the load beams. The longer load beams makes it possible to switch the shutters in the array at reduced voltages. In particular, the narrower shutter 1302 enables portions of the actuators 1304 and 1306 of the shutter assemblies 1300 to interleave with the gaps between actuators 1302 and 1304 of neighboring shutter assemblies 1300. The interleaved arrangement of FIG. 20B can nevertheless still be mapped onto a square arrangement of rows and columns, which is the common pixel configuration for textual displays.

The tiling or pixel arrangements for shutter assemblies need not be limited to the constraints of a square array. Dense tiling can also be achieved using rectangular, rhombehedral, or hexagonal arrays of pixels, all of which find applications, for example in video and color imaging displays.

FIG. 21 is a cross sectional view of a display apparatus 2100 incorporating dual compliant electrode actuator-based shutter assemblies 2102, according to an illustrative embodiment of the invention. The shutter assemblies 2102 are disposed on a glass substrate 2104. A rear-facing reflective layer, reflective film 2106, disposed on the substrate 2104 defines a plurality of surface apertures 2108 located beneath the closed positions of the shutters 2110 of the shutter assemblies 2102. The reflective film 2106 reflects light not passing through the surface apertures 2108 back towards the rear of the display apparatus 2100. The reflective aperture layer 2106 can be a fine-grained metal film without inclusions formed in thin film fashion by a number of vapor deposition techniques including sputtering, evaporation, ion plating, laser ablation, or chemical vapor deposition. In another implementation, the rear-facing reflective layer 2106 can be formed from a mirror, such as a dielectric mirror. A dielectric mirror is fabricated as a stack of dielectric thin films which alternate between materials of high and low refractive index.

The display apparatus 2100 includes an optional diffuser 2112 and/or an optional brightness enhancing film 2114 which separate the substrate 2104 from a backlight 2116. The backlight 2116 is illuminated by one or more light sources 2118. The light sources 2118 can be, for example, and without limitation, incandescent lamps, fluorescent lamps, lasers, or light emitting diodes. A front-facing reflective film 2120 is disposed behind the backlight 2116, reflecting light towards the shutter assemblies 2102. Light rays from the backlight that do not pass through one of the shutter assemblies 2102 will be returned to the backlight and reflected again from the film 2120. In this fashion light that fails to leave the display to form an image on the first pass can be recycled and made available for transmission through other open apertures in the array of shutter assemblies 2102. Such light recycling has been shown to increase the illumination efficiency of the display.

In one implementation the light sources 2118 can include lamps of different colors, for instance, the colors red, green, and blue. A color image can be formed by sequentially illuminating images with lamps of different colors at a rate sufficient for the human brain to average the different colored images into a single multi-color image. The various color-specific images are formed using the array of shutter assemblies 2102. In another implementation, the light source 2118 includes lamps having more than three different colors. For example, the light source 2118 may have red, green, blue and white lamps or red, green, blue, and yellow lamps.

A cover plate 2122 forms the front of the display apparatus 2100. The rear side of the cover plate 2122 can be covered with a black matrix 2124 to increase contrast. The cover plate 2122 is supported a predetermined distance away from the shutter assemblies 2102 forming a gap 2126. The gap 2126 is maintained by mechanical supports or spacers and/or by an epoxy seal 2128 attaching the cover plate 2122 to the substrate 2104. The epoxy 2128 should have a curing temperature preferably below about 200° C., it should have a coefficient of thermal expansion preferably below about 50 ppm per degree C. and should be moisture resistant. An exemplary epoxy 2128 is EPO-TEK B9021-1, sold by Epoxy Technology, Inc.

The epoxy seal 2128 seals in a working fluid 2130. The working fluid 2130 is engineered with viscosities preferably below about 10 centipoise and with relative dielectric constant preferably above about 2.0, and dielectric breakdown strengths above about 10⁴ V/cm. The working fluid 2130 can also serve as a lubricant. Its mechanical and electrical properties are also effective at reducing the voltage necessary for moving the shutter between open and closed positions. In one implementation, the working fluid 2130 preferably has a low refractive index, preferably less than about 1.5. In another implementation the working fluid 2130 has a refractive index that matches that of the substrate 2104. In another implementation the working fluid 2130 has a refractive index greater than that of the substrate. In another implementation the working fluid has a refractive index greater than 2.0. Suitable working fluids 2130 include, without limitation, de-ionized water, methanol, ethanol, silicone oils, fluorinated silicone oils, dimethylsiloxane, polydimethylsiloxane, hexamethyldisiloxane, and diethylbenzene.

In another implementation, the working fluid 2130 is a hydrophobic liquid with a high surface wetting capability. Preferably, its wetting capabilities are sufficient to wet the front as well as the rear surfaces of the shutter assemblies 2102. Hydrophobic fluids are capable of displacing water from the surfaces of shutter assemblies 2130. In another implementation, the working fluid 2130 contains a suspension of particles with diameters in the range of 0.5 to 20 microns. Such particles scatter light to increase the viewing angle of a display. In another implementation the working fluid 2130 contains dye molecules in solution for absorbing some or all frequencies of visible light to increase the contrast of the display.

Illustrative methods and materials for forming the reflective apertures 2106 on the same substrate as the shutter assemblies 2102 are disclosed in co-owned U.S. patent application Ser. No. 11/361,785, filed Feb. 23, 2006, incorporated herein by reference.

A sheet metal or molded plastic assembly bracket 2132 holds the cover plate 2122, shutter assemblies 2102, the substrate 2104, the backlight 2116 and the other component parts together around the edges. The assembly bracket 2132 is fastened with screws or indent tabs to add rigidity to the combined display apparatus 2100. In some implementations, the light source 2118 is molded in place by an epoxy potting compound.

Display apparatus 2100 is referred to as the MEMS-up configuration, wherein the MEMS based light modulators are formed on a front surface of substrate 2104, i.e. the surface that faces toward the viewer. The shutter assemblies 2102 are built directly on top of the reflective aperture layer 2106. In an alternate embodiment of the invention, referred to as the MEMS-down configuration, the shutter assemblies are disposed on a substrate separate from the substrate on which the reflective aperture layer is formed. The substrate on which the reflective aperture layer is formed is referred to herein as the aperture plate. For the MEMS-down configuration, the substrate on which the MEMS-based light modulators are formed takes the place of cover plate 2122 in display apparatus 2100. In the MEMS-down configuration, the substrate that carries the MEMS-based light modulators is oriented such that the MEMS-based light modulators are positioned on the rear surface of the top substrate, i.e. the surface that faces away from the viewer and toward the back light 2116. The MEMS-based light modulators are thereby disposed directly opposite to and across a gap from the reflective aperture layer. Display apparatus corresponding to the MEMS-down configuration are described further in U.S. patent application Ser. No. 11/361,785, filed Feb. 23, 2006 and U.S. patent application Ser. No. 11/528,191, filed Sep. 26, 2006, both of which are incorporated herein by reference.

In various embodiments, it is advantageous for the shutters used in shutter assemblies to overlap the apertures to which they correspond, when the shutters are in the closed position. FIGS. 22A and 22B are top views of a shutter assembly 2200, similar to the shutter assembly 800 of FIG. 8, in opened and closed positions, respectively, illustrating such an overlap. The shutter assembly 2200 includes a shutter 2202 supported over a reflective aperture layer 2204 by anchors 2206 via portions of opposing actuators 2208 and 2210. The shutter assembly 2200 is suitable for inclusion in an array of light modulators included in a display apparatus.

The shutter 2202 includes three shutter apertures 2212, through which light can pass. The remainder of the shutter 2202 obstructs the passage of light. In various embodiments, the side of the shutter 2202 facing the reflective aperture layer 2204 is coated with a light absorbing material or a reflective material to absorb or reflect, respectively, obstructed light.

The reflective aperture layer 2204 is deposited on a transparent substrate, preferably formed from plastic or glass. The reflective aperture layer 2204 can be formed from a film of metal deposited on the substrate, a dielectric mirror, or other highly reflective material or combination of materials. The reflective aperture layer 2204 has a set of apertures 2214 formed in it to allow light to pass through the apertures, from the transparent substrate, towards the shutter 2202. The reflective aperture layer 2204 has one aperture corresponding to each shutter aperture 2212. For example, for an array of light modulators including shutter assemblies 2200, the reflective aperture layer includes three apertures 2214 for each shutter assembly 2200. Each aperture has at least one edge around its periphery. For example, the rectangular apertures 2214 have four edges. In alternative implementations in which circular, elliptical, oval, or other curved apertures are formed in the reflective aperture layer 2204, each aperture may have only a single edge.

In FIG. 22A, the shutter assembly 2200 is in an open state. Actuator 2208 is in an open position, and actuator 2210 is in a collapsed position. Apertures 2214 are visible through the shutter apertures 2212. As visible, the shutter apertures 2212 are larger in area than the apertures 2214 formed in the reflective aperture layer 2204. The size differential increases the range of angles at which light can pass through the shutter apertures 2212 towards an intended viewer.

In FIG. 22B, the shutter assembly is a closed state. Actuator 2208 is in a collapsed position and actuator 2210 is in an open position. Light blocking portions of the shutter 2202 cover the apertures 2214 in the reflective aperture layer 2204. The light blocking portions of the shutter 2202 overlap the edges of the apertures 2214 in the reflective aperture layer 2204 by a predefined overlap 2216. In some implementations, even when a shutter is in a closed state, some light, at angles far from an axis normal to the shutter 2202, may leak through the apertures 2214. The overlap included in shutter assembly 2200 reduces or eliminates this light leakage. While, as depicted in FIG. 22B, the light blocking portions of shutter 2202 overlap all four edges of the aperture, having the light blocking portions of shutter 2202 overlap even one of the edges reduces light leakage.

FIG. 23A-23C are cross-sectional views of various configurations of the shutter assembly 2200 in relation to the transparent substrate on which the reflective aperture layer 2204 is formed. The cross sectional views correspond to the line labeled B-B′ on FIGS. 22A and 22B. For purposes of illustration, the shutter 2202 is illustrated in FIGS. 23A-23C as having only a single shutter aperture 2323 and two light blocking portions 2324.

FIG. 23A is a cross section of a first configuration of a display apparatus 2300 including a shutter assembly 2301 similar to that depicted in FIG. 22 in the closed state taken across line B-B′, according to an illustrative embodiment of the invention. In the first configuration, the shutter assembly 2301 is formed on a reflective aperture layer 2302. The reflective aperture layer 2302 is formed from a thin metal film deposited on a transparent substrate 2304. Alternately, the reflective aperture layer 2302 can be formed from a dielectric mirror, or other highly reflective material or combination of materials. The reflective aperture layer 2302 is patterned to form apertures 2306. The transparent substrate 2304 is positioned proximate a light guide 2308. The transparent substrate 2304 and the light guide 2308 are separated by a gap 2309 filled with a fluid, such as air. The refractive index of the fluid is preferably less than that of the light guide 2308. Suitable light guides 2308 for display apparatus 2300 are described further in U.S. patent application Ser. No. 11/528,191, the entirety of which is herein incorporated by reference. The display apparatus 2300 also includes a front-facing rear reflective layer 2310 positioned adjacent the rear side of the light guide 2308.

The shutter assembly 2301 includes the shutter 2314 supported proximate to the reflective aperture layer 2302 by anchors 2316 via portions of opposing actuators 2318 and 2320. The anchors 2316 and actuators 2318 and 2320 suspend the shutter 2314 at about a constant distance H1 (measured from the bottom of the shutter 2314) over the reflective aperture layer 2302. In addition, the display apparatus 2300 includes a cover plate 2311 supported over the transparent substrate 2304 by spacer posts 2312. The spacer posts 2312 keep the cover plate at about a second constant distance H2 away from the top of the shutter 2314. The substrates 2304 and 2311 can be made of a substantially rigid material, such as glass, in which case a relatively low density of spacers 2312 may be used to maintain the desired spacing H2. For example, with rigid substrates, the display apparatus 2300, in one implementation, includes one spacer 2312 for every 4 pixels, though other densities, both higher and lower, may also be employed. In an alternative implementation either substrate 2304 or 2311 can be made of a flexible material, such as plastic, in which case it is preferable to have a higher density of spacers 2312, for example, one spacer 2312 within or between each pixel in the array.

The gap between the cover plate 2311 and the transparent substrate 2304 is filled with a working fluid 2322, such as working fluid 2130, described above. The working fluid 2322 preferably has a refractive index greater than that of the transparent substrate 2304. In another implementation the working fluid has a refractive index greater than 2.0. In another implementation the working fluid 2322 has a refractive index that is equal to or less than the index of refraction of the transparent substrate 2304.

As indicated above, the shutter assembly 2301 is in the closed state. Light blocking portions 2324 of the shutter 2314 overlap the edges of the apertures 2306 formed in the reflective aperture layer 2302. The light blocking properties of shutter 2314 are improved when the gap between the shutter and the aperture, i.e. the distance H1, is made as small as possible. In one implementation, H1 is less than about 100 μm. In another implementation, H1 is less than about 10 μm. In still another implementation, H1 is about 1 μm. In an alternative embodiment the distance H1 is greater than 0.5 mm, but remains smaller than the display pitch. The display pitch is defined as the distance between pixels (measured center to center), and in many cases is established as the distance between apertures, such as apertures 2306, measured center to center, in the rear-facing reflective layer 2302.

The size of the overlap W1 is preferably proportional to the distance H1. While the overlap W1 may be smaller, preferably the overlap W1 is greater than or equal to the distance H1. In one implementation the overlap W1 is greater than or equal to 1 micron. In another implementation the overlap W1 is between about 1 micron and 10 microns. In another implementation the overlap W1 is greater than 10 microns. In one particular implementation, the shutter 2314 is about 4 μm thick. H1 is about 2 μm, H2 is about 2 μm, and W1>=2 μm. By having the overlap W1 being greater than or equal to H1, if the shutter assembly 2301 is in the closed state as depicted in FIG. 23A, most light having a sufficient angle to escape the light guide 2308 through the apertures 2306 impacts the light blocking portions 2324 of the shutter 2314, thereby improving the contrast ratio of the display apparatus 2300.

H2 is preferably about the same distance as H1. The spacer posts 2312 are preferably formed from a polymer material that is lithographically patterned, developed, and/or or etched into cylindrical shapes. The height of the spacer is determined by the cured thickness of the polymer material. Methods and materials for formation of spacers 2312 are disclosed in co-owned U.S. patent application Ser. No. 11/361,785, filed Feb. 23, 2006, incorporated herein by reference. In an alternative embodiment the spacer 2312 can be formed from a metal which is electrochemically deposited into a mold made from a sacrificial material.

FIG. 23B is a cross section of a second configuration of a display apparatus 2340 including a shutter assembly 2341 similar to that depicted in FIG. 22 in the closed state, according to an illustrative embodiment of the invention. This second configuration is referred to as the MEMS-down configuration, in which the reflective aperture layer 2344 is formed on a transparent substrate called the aperture plate 2346, which is distinct from the light modulator substrate 2342 to which shutter assembly 2341 is anchored. The shutter assembly includes a shutter 2354 having light blocking portions 2362 and shutter apertures 2363 formed therein. Like the aperture plate 2346, the light modulator substrate 2342 is also transparent. The two substrates 2342 and 2346 are separated by a gap. The two substrates 2342 and 2346 are aligned during assembly such that a one to one correspondence exists, as indicated in FIG. 22, between each of the apertures 2347 and the light blocking portions 2362 of shutter 2354 when that shutter is in the closed position, and/or between the apertures 2347 and the shutter apertures 2363 when that shutter is in the open position. In alternative embodiments, the correspondence between apertures and either light blocking portions 2362 or shutter apertures 2363 of a shutter 2354 is a one to many or many to one correspondence.

In the MEMS-down display apparatus 2340, the shutter assembly 2341 is formed on the rear-facing surface of the light modulator substrate 2342, i.e. on the side which faces the light guide 2348. In display apparatus 2340, the aperture plate 2346 is positioned between the light modulator substrate 2342 and the light guide 2348 The reflective aperture layer 2344 is formed from a thin metal film deposited on the front-facing surface of transparent aperture plate 2346. The reflective aperture layer 2344 is patterned to form apertures 2347. In another implementation, the reflective layer 2344 can be formed from a mirror, such as a dielectric mirror. A dielectric mirror is fabricated from a stack of dielectric thin films with different refractive indices, or from combinations of metal layers and dielectric layers.

The aperture plate 2346 is positioned proximate to a backlight or light guide 2348. The aperture plate 2346 is separated from the light guide 2348 by a gap 2349 filled with a fluid, such as air. The refractive index of the fluid is preferably less than that of the light guide 2348. Suitable backlights 2348 for display apparatus 2340 are described further in U.S. patent application Ser. No. 11/528,191, the entirety of which is herein incorporated by reference. The display apparatus 2340 also includes a front-facing rear reflective layer 2350 positioned adjacent the rear side of the backlight 2348. The front-facing reflective layer 2350 combined with the rear-facing reflective layer 2344 forms an optical cavity which promotes recycling of light rays which do not initially pass through apertures 2347. The shutter assembly 2341 includes the shutter 2354 supported proximate to the transparent substrate 2342 by anchors 2356 via portions of opposing actuators 2358 and 2360. The anchors 2356 and actuators 2358 and 2360 suspend the shutter 2354 at about a constant distance H4 (measured from the top of the shutter 2354) below the light modulator substrate 2342. In addition, display apparatus includes spacer posts 2357, which support the light modulator substrate 2342 over the aperture plate 2346. The spacer posts 2357 keep the light modulator substrate 2342 at about a second constant distance H5 away from the aperture plate 2346, thereby keeping the bottom surface of shutter 2354 at a third about constant distance H6 above the reflective aperture layer 2344. The spacer posts 2357 are formed in a fashion similar to those of spacers 2312.

The gap between the light modulator substrate 2342 and the aperture plate 2346 is filled with a working fluid 2352, such as working fluid 2130, described above. The working fluid 2352 preferably has a refractive index greater than that of the transparent aperture plate 2346. In another implementation the working fluid has a refractive index greater than 2.0. In another implementation the working fluid 2352 preferably has a refractive index that is equal to or less than the index of refraction of the aperture plate 2346.

As indicated above, the shutter assembly 2341 is in the closed state. Light blocking portions 2362 of the shutter 2354 overlap the edges of the apertures 2347 formed in the reflective aperture layer 2344. The size of the overlap W2 is preferably proportional to the distance H6. While the overlap W2 may be smaller, preferably the overlap W2 is greater than or equal to the distance H6. In one implementation, H6 is less than about 100 μm. In another implementation, H6 is less than about 10 μm. In still another implementation, H6 is about 1 μm. In an alternative embodiment the distance H6 is greater than 0.5 mm, but remains smaller than the display pitch. The display pitch is defined as the distance between pixels (measured center to center), and in many cases is established as the distance between the centers of apertures in the rear-facing reflective layer, such as apertures 2347. H4 is preferably about the same distance as H6. In one particular implementation, the shutter 2354 is about 4 μm thick, H6 is about 2 μm, H4 is about 2 μm, H5 is about 8 μm and W2>=2 μm. By having the overlap W2 being greater than or equal to H6, if the shutter assembly 2341 is in the closed state as depicted in FIG. 23B, most light having a sufficient angle to escape the backlight 2348 through the apertures 2347 impacts the light blocking portions 2362 of the shutter 2354, thereby improving the contrast ratio of the display apparatus 2340.

FIG. 23C is a cross section of a third configuration of a display apparatus 2370 including a shutter assembly 2371 similar to that depicted in FIG. 22 in the closed state, according to an illustrative embodiment of the invention. In comparison to the second configuration of the display apparatus 2340 described above, the display apparatus 2370 is designed to account for minor misalignments that may occur during the aligning and bonding of a light modulator substrate 2372 (similar to light modulator substrate 2342) on which a shutter assembly 2371 is formed to an aperture plate 2374 (similar to the aperture plate 2346) on which a reflective aperture layer 2376 is deposited. To address this potential issue, the display apparatus 2370 includes an additional layer of light absorbing material 2377, deposited on the light modulator substrate 2372. The light absorbing material 2377 may be part of a black mask, though at least some of the light absorbing material 2377 is preferably located in the interior of a pixel to which the shutter assembly 2371 corresponds. The light absorbing material 2377 absorbs light 2378 that would otherwise pass through the light modulator substrate 2372 while the shutter 2382 is in the closed state. Additional light absorbing material 2377 may be deposited on the front side of reflective aperture layer 2376 to absorb light, for example light 2380 deflected from a shutter 2382.

FIG. 23D is a cross section of a fourth configuration of a display apparatus 2390 including a shutter assembly 2385 similar to that depicted in FIG. 22 in the closed state, according to an illustrative embodiment of the invention. In comparison to the second configuration of shutter assembly 2354 described above, the shutter assembly 2385 is fabricated according to a different process resulting in different cross sectional thicknesses for some of its members. The resulting shutter 2393 is referred to herein as a corrugated shutter. The design guidelines for gap distances, e.g. H8 and H10, and for the overlap parameter W4, however, are preferably unchanged from the corresponding gap distances and the overlap parameters described above. The display apparatus 2390 includes a transparent light modulator substrate 2386, oriented in the MEMS down configuration, and to which the shutter assembly 2385 is attached. The display apparatus 2390 also includes a transparent aperture plate 2387 on which a rear-facing reflective aperture layer 2388 is deposited. The display apparatus 2390 includes a fluid 2389 which fills the gap between substrates 2386 and 2387. The fluid 2389 preferably has a refractive index higher than that of the aperture plate 2387. The display apparatus also includes a backlight 2348 along with front-facing reflective layer 2350.

The shutter assembly 2385 is in the closed state. Light blocking portions 2391 of the corrugated shutter 2393 overlap the edges of apertures 2394 formed in the reflective aperture layer 2388. The corrugated shutter 2393 is comprised of two connected flat plate sections: section 2391 which is oriented horizontally and section 2392 which is oriented vertically. Each flat plate 2391 and 2392 is comprised of thin film materials with thicknesses in the range of 0.2 to 2.0 μm. In a particular embodiment the thickness of the horizontal section 2391 is 0.5 μm. The vertical section 2392 provides a stiffness to the corrugated shutter 2393 and a height which matches that of actuator 2358 without requiring the deposition of a bulk materials thicker than about 2 am. Methods and materials for formation of shutters with a corrugated and/or three dimensional structures are disclosed in co-owned U.S. patent application Ser. No. 11/361,785, filed Feb. 23, 2006, incorporated herein by reference.

Similar to dimensions described for display apparatus 2340, in a particular example the dimensions of H8, H9, and H10 of display apparatus 2390 can be 2, 8, and 2 μm respectively. The overlap W4 is preferably greater than or equal to the distance H10. In another example, the distance H10 and the overlap W4 can be >=1 μm. Using the materials and methods for a corrugated shutter 2393, however, the thickness of section 2391 can be as thin as 0.5 μm. By having the overlap W4 greater than or equal to H10, if the shutter assembly 2385 is in the closed state as depicted in FIG. 23A, most light having a sufficient angle to escape the backlight 2348 through the apertures 2394 impacts the light blocking portions 2391 of the shutter 2393, thereby improving the contrast ratio of the display apparatus 2390.

FIG. 24 is a cross sectional view of a first electrowetting-based light modulation array 2400, according to an illustrative embodiment of the invention. The light modulation array 2400 includes a plurality of electrowetting-based light modulation cells 2402 a-2402 d (generally “cells 2402”) formed on an optical cavity 2404. The light modulation array 2400 also includes a set of color filters 2406 corresponding to the cells 2402.

Each cell 2402 includes a layer of water (or other transparent conductive or polar fluid) 2408, a layer of light absorbing oil 2410, a transparent electrode 2412 (made, for example, from indium-tin oxide) and an insulating layer 2414 positioned between the layer of light absorbing oil 2410 and the transparent electrode 2412. Illustrative implementation of such cells are described further in U.S. Patent Application Publication No. 2005/0104804, published May 19, 2005 and entitled “Display Device,” incorporated herein by reference. In the embodiment described herein, the transparent electrode 2412 takes up only a portion of a rear surface of a cell 2402.

The remainder of the rear surface of a cell 2402 is formed from a reflective aperture layer 2416 that forms the front surface of the optical cavity 2404. The rear-facing reflective layer 2416 is patterned to form apertures, which in the embodiment of cell 2402 are coincident with the transparent electrode 2412. Preferably, when in the closed position, the layer of light absorbing oil 2410 overlaps one or more edges of its corresponding aperture in the reflective aperture layer 2416. The reflective aperture layer 2416 is formed from a reflective material, such as a reflective metal or a stack of thin films forming a dielectric mirror. For each cell 2402, an aperture is formed in the reflective aperture layer 2416 to allow light to pass through. In an alternate embodiment, the electrode 2412 for the cell is deposited in the aperture and over the material forming the reflective aperture layer 2416, separated by another dielectric layer.

The remainder of the optical cavity 2404 includes a light guide 2418 positioned proximate the reflective aperture layer 2416, and a second reflective layer 2420 on a side of the light guide 2418 opposite the reflective aperture layer 2416. A series of light redirectors 2421 are formed on the rear surface of the light guide, proximate the second reflective layer. The light redirectors 2421 may be either diffuse or specular reflectors. One of more light sources 2422 inject light 2424 into the light guide 2418.

In an alternate implementation the light sources 2422 can include lamps of different colors, for instance, the colors red, green, and blue. A color image can be formed by sequentially illuminating images with lamps of different colors at a rate sufficient for the human brain to average the different colored images into a single multi-color image. The various color-specific images are formed using the array of electrowetting modulation cells 2402. In another implementation, the light source 2422 includes lamps having more than three different colors. For example, the light source 2422 may have red, green, blue and white lamps or red, green, blue, and yellow lamps.

In an alternative implementation, the cells 2402 and the reflective aperture layer 2416 are formed on an additional light modulator substrate which is distinct from light guide 2418 and separated from it by a gap. (See for example the light modulator substrate 2513 of FIG. 25.) In yet another implementation, a layer of material with a refractive index less than that of the light guide 2418 is interposed between the reflective aperture layer 2416 and the light guide 2418. The layer of material with lower refractive index may help to improve the uniformity of light emitted from the light guide 2418.

In operation, application of a voltage to the electrode 2412 of a cell (for example, cell 2402 b or 2402 c) causes the light absorbing oil 2410 in the cell to collect in one portion of the cell 2402. As a result, the light absorbing oil 2410 no longer obstructs the passage of light through the aperture formed in the reflective aperture layer 2416 (see, for example, cells 2402 b and 2402 c). Light escaping the backlight at the aperture is then able to escape through the cell and through a corresponding color (for example, red, green, or blue) filter in the set of color filters 2406 to form a color pixel in an image. When the electrode 2412 is grounded, the light absorbing oil 2410 covers the aperture in the reflective aperture layer 2416, absorbing any light 2424 attempting to pass through it (see for example cell 2402 a).

The area under which oil 2410 collects when a voltage is applied to the cell 2402 constitutes wasted space in relation to forming an image. This area cannot pass light through, whether a voltage is applied or not, and therefore, without the inclusion of the reflective portions of reflective apertures layer 2416, would absorb light that otherwise could be used to contribute to the formation of an image. However, with the inclusion of the reflective aperture layer 2416, this light, which otherwise would have been absorbed, is reflected back into the light guide 2420 for future escape through a different aperture.

FIG. 25 is a cross sectional view of a second electrowetting-based light modulation array 2500, according to an illustrative embodiment of the invention. The second electrowetting-based light modulation array 2500 includes three sub-arrays 2501 a, 2501 b, and 2501 c of colored electrowetting-based light modulation cells 2502 (generally “cells 2502”), positioned on top of one another. Each cell 2502 includes a transparent electrode 2504, and a colored oil 2506 separated by an insulator 2508. In one implementation, the oil 2506 in the cells 2502 of sub-array 2501 a is colored cyan, the oil 2506 in the cells 2502 of sub-array 2501 b is colored yellow, and the oil 2506 in the cells 2502 of sub-array 2501 c is colored magenta. The cells 2502 in sub-array 2501 a and the cells 2502 of sub-array 2501 b share a common layer of water 2520. The cells 2502 of sub-array 2501 c include their own layer of water 2520.

The electrowetting-based light modulation array 2500 includes a light-recycling optical cavity 2510 coupled to the three sub-arrays 2501 a-2501 c. The optical cavity 2510 includes a light guide 2512 and a light modulator substrate 2513, separated from the light guide 2512 by a gap 2515. The front surface of the light modulator substrate 2513 includes a rear-facing reflective aperture layer 2514. The reflective aperture layer 2514 is formed from a layer of metal or a stack of thin films forming a dielectric mirror. Apertures 2516 are patterned into the reflective aperture layer beneath the cells 2502 of the sub-arrays 2501 a-2501 c to allow light to escape the light guide and pass through the sub-arrays 2501 a-2501 c to form an image. The transparent electrodes 2504 of cells 2502 are formed over the top of the reflective aperture layer 2514.

The substrates, i.e., light guide 2512 and modulator substrate 2513, are separated by a gap 2515 filled with a fluid, such as air. The refractive index of the fluid is less than that of the light guide 2512. A front-facing reflective layer 2518 is formed on, or positioned proximate to, the opposite side of the light guide 2512. The light modulation array 2500 includes at least one light source 2522 for injecting light into the light guide 2512. Suitable light guides 2618 for display apparatus 2600 are described further in U.S. patent application Ser. No. 11/528,191, the entirety of which is herein incorporated by reference.

FIG. 26 is a cross sectional view of a third electrowetting-based light modulation array 2600, according to an illustrative embodiment of the invention. The light modulation array 2600 includes a plurality of electrowetting-based light modulation cells 2602 a-2602 c (generally “cells 2602”) formed on an optical cavity 2604. The light modulation array 2600 also includes a set of color filters 2606 corresponding to the cells 2602.

While the array 2400 might be considered an example of an array in a MEMS-up configuration, the array 2600 is an example of an electrowetting-based array assembled in a MEMS-down configuration. Each cell 2602 includes a layer of water (or other transparent conductive or polar fluid) 2608, a layer of light absorbing oil 2610, a transparent electrode 2612 (made, for example, from indium-tin oxide) and an insulating layer 2614 positioned between the layer of light absorbing oil 2610 and the transparent electrode 2612. In the MEMS-down configuration of light modulator array 2600, however, both the insulating layer 2614 and the transparent electrode 2612 are disposed on a light modulator substrate 2630 distinct from an aperture plate 2632. Like the light modulator substrate 2630, the aperture plate 2632 is also a transparent substrate. The light modulator substrate 2630 is the topmost substrate and is oriented such that control electrodes such as transparent electrode 2612 are disposed on the rear surface of substrate 2630, i.e. the surface that faces away from the viewer and toward the light guide. In addition to transparent electrode 2612, the rear surface of light modulator substrate 2630 can carry other common components of a switching or control matrix for the modulator array, including without limitation, row electrodes, column electrodes, transistors for each pixel and capacitors for each pixel. The electrodes and switching components formed on light modulator substrate 2630, which govern the actuation of light modulators in the array, are disposed opposite to and across a gap 2636 from a reflective aperture layer 2616, disposed on the front surface of aperture plate 2632. The gap 2636 is filled with the electrowetting fluid components water 2608 and oil 2610.

The reflective aperture layer 2616 is deposited on transparent substrate 2632, preferably formed from plastic or glass. The reflective aperture layer 2616 can be formed from a film of metal deposited on the substrate, a dielectric mirror, or other highly reflective material or combination of materials. The reflective aperture layer 2616 is a rear-facing reflective layer, forming the front surface of optical cavity 2604. The reflective aperture layer 2616 has a set of apertures 2617 formed in it to allow light to pass through the apertures toward the electrowetting fluid components 2608 and 2610. Optionally, the aperture plate 2632 includes a set of color filters 2606 deposited on the top surface of reflective aperture 2616 and filling the apertures 2617.

The aperture plate 2632 is positioned between the light modulator substrate 2630 and the light guide 2618. The substrates 2632 and 2618 are separated from each other by a gap 2634 filled with a fluid (such as air). The refractive index of the fluid is less than that of the light guide 2618. Suitable light guides 2618 for display apparatus 2600 are described further in U.S. patent application Ser. No. 11/528,191, the entirety of which is herein incorporated by reference. The optical cavity 2604 also includes substrates 2632, 2618, and the front-facing rear reflective layer 2620 positioned adjacent the rear side of the light guide 2618. One or more light sources 2622 inject light into the light guide 2618.

The reflective aperture layer 2616 has one aperture 2617 corresponding to each light modulator cell 2602 in the array 2600. Similarly, the light modulator substrate 2630 has one transparent electrode 2612 or one set of pixel transistors and capacitors for each light modulator cell 2602. The substrates 2630 and 2632 are aligned during assembly to ensure that corresponding apertures 2617 are positioned where light will not be obstructed by the oil 2610 when cells are actuated or held in the open state, e.g. cell 2602 b.

Fabrication of an Aperture Plate

The aperture plate 2700 of FIG. 27 illustrates the detailed structures within one implementation of an aperture plate, such as aperture plate 2346, 2374, 2387, or 2632 according to an illustrative embodiment of the invention. The aperture plate 2700 includes a substrate 2702, a dielectrically enhanced metal mirror 2704, a light absorbing layer 2706, and a spacer post 2708. The dielectrically enhanced metal mirror and the light absorbing layer have been patterned into apertures 2709.

The substrate 2702 is preferably a transparent material, for example glass or plastic. The dielectrically enhanced metal mirror 2704 is comprised of a 5-layer stack of materials including, in order from the substrate up, a thin film of Si₃N₄ 2710, a thin film of SiO₂ 2712, another thin film of Si₃N₄ 2710, another thin film of SiO₂, 2712, and a thin film of aluminum 2714. The relative thicknesses and preferred refractive indices of these layers are given in Table 1:

TABLE 1 Film Thicknesses and Refractive Indices for a Dielectrically Enhanced Metal Mirror Thin film material Thickness Refractive index 5. Aluminum 200 nm or less NA 4. SiO₂  88 nm 1.46 3. Si₃N₄  64 nm 2.0 2. SiO₂  88 nm 1.46 1. Si₃N₄  64 nm 2.0

The light absorbing layer 2706 can be formed from a thin film of black chrome, which is a composite of chromium metal particles suspended in an oxide or nitride matrix. Examples include Cr particles in a Cr₂O₃ matrix or Cr particles in an SiO₂ matrix. In other implementations black chrome can be formed from a thin metal film of chromium upon which a thin film of CrOx (a sub-oxide of chromium) has been either grown or deposited. A preferred thickness for the black chrome is 150 nm.

The aperture windows 2709 can be patterned from the thin film stack of materials 2704 and 2706 by processes known in the art such as photolithography and etch or by photolithography and lift-off. In the etch process a layer of photoresist is added to the top of the thin film stack and then exposed to UV light through a mask. After developing the aperture pattern in the exposed layer of photoresist, the whole stack is etched in the region of apertures 2709 down to the substrate 2702. Such etching may be accomplished by immersion in wet chemicals, by a dry plasma or ion beam etch, or any combination of the above. In the lift-off process the layer of photoresist is added to the glass before deposition of the thin film stack, the resist being developed into a pattern that is a reverse of the etch mask pattern. The thin film stack is then deposited over the top of the photoresist, such that the thin film stack makes contact to the glass everywhere except in the regions of the apertures 2709. After deposition of the thin film stack is complete, the substrate is dipped into a bath of chemicals that dissolves or lifts-off the photoresist as well as any thin film materials that were deposited on top of the photoresist.

The spacer post 2708 is formed from a photo-imagable polymer such as such as a photo-imagable epoxy (in particular a novolac epoxy) or a photo-imagable polyimide material. Other polymer families that can be prepared in photo-imagable form and are useful for this application include polyarylene, parylene, benzocyclobutane, perfluorocyclobutane, silsequioxane, and silicone polymers. A particular photo-imagable resist useful for the spacer application is the Nano SU-8 material available from Microchem Corporation, headquartered in Newton, Mass.

The polymer spacer material is initially deposited as a thick film on top of the thin film stack 2704 and 2706 after the apertures 2709 have been patterned. The photo-imagable polymer is then exposed to UV light through a mask. Alignment marks help to ensure that the resultant spacers 2708 are located correctly with respect to apertures 2709. For instance, alignment fiducials can be formed on the periphery of the display during the process of etching the apertures 2709. These fiducials are then aligned to a corresponding set of fiducials on the exposure mask to ensure a correct location of spacers 2708. A developing process is then effective at removing all of the polymer except where it was exposed to the UV light. In an alternate method, the features on the exposure mask may be aligned directly to display features on the substrate 2702, such as the apertures 2709.

In the particular implementation described with respect to display apparatus 2340, the spacer posts can be 8 microns tall. In other implementations spacer heights may range from about 2 microns to about 50 microns. When cross sectioned in the plane of the substrate 2702, the spacers may take regular shapes such as a cylinder or a rectangle with widths in the range of 2 to 50 microns. Alternately, they can have complex irregular cross sections which are designed to maximize the contact area of the spacer while fitting between other structures on the substrate, such as apertures 2709. In a preferred implementation the spacer size, shape and placement is determined so that the spacers do not interfere with the movement of the shutters, such as shutters 2354 or other MEMS components, such as actuators 2358 in display apparatus 2340.

In another embodiment, the spacer post 2708 is not provided as a polymer material but is instead composed of a heat re-flowable joining material, such as a solder alloy. Exemplary heat re-flowable materials are described below with respect to FIG. 29B. The solder alloy can pass through a melting or re-flow step which allows the solder alloy to wet or bond to a mating surface on the opposing substrate. The solder alloy therefore performs an additional function as a joining material between an aperture plate, such as aperture plate 2346 and a modulator substrate, such as substrate 2342. Because of the reflow process, the solder alloy typically relaxes to an oblate shape referred to as the solder bump. A predetermined spacing between substrates can be maintained through control over the average volume of material in the solder bump. Solder bumps can be applied to aperture plate 2700 by means of thin film deposition, by thick film deposition through a stencil mask, or by electroplating.

In another embodiment, the aperture plate 2700 can be subjected to a sandblasting treatment after the steps of forming the optical layers 2704 and 2708. The sandblasting has the effect of roughening the substrate surface selectively in the regions of the aperture 2709. A roughened surface at aperture 2709 behaves as an optical diffuser which can provide the benefits of a wider viewing angle for the display. In another embodiment, a diffusing surface at aperture 2709 is provided by means of an etching process, where the etch is selectively applied in the regions of apertures 2709 after exposure of photoresist to a photomask. Etch pits or trenches can be created through proper design of the photomask, and the sidewall angles or depths of the pits or trenches can be controlled by means of either a wet or dry etch process. In this fashion optical structures with controlled degrees of diffusive broadening can be created. In this fashion anisotropic diffusers can be created at the substrate surface which deflect light along a preferred optical axis, creating elliptical and/or multi-directional cones of emitted light.

In another embodiment, an etched trench can be provided in substrate 2702 that substantially surrounds the display along the periphery of the array of apertures 2709 (i.e. around the periphery of the active display region). The etched trench performs as a mechanical locating structure for restricting the motion or flowing of an adhesive, such as adhesive 2128, used to seal aperture plate 2700 to an opposing substrate.

Further details regarding the materials and processes described above can be found in U.S. patent application Ser. No. 11/361,785, filed Feb. 23, 2006, incorporated herein by reference. For example, that application includes additional materials and processing methodologies regarding the formation of dielectrically enhanced metal mirrors with apertures, light absorbing layers, and spacer posts. Although dielectric mirrors and spacers are described in that application in the context of an integrated (for example MEMS-up) display design, it will be understood that similar processes can be adapted to the fabrication of an aperture plate, such as aperture plate 2700.

In some implementations of the aperture plate 2700, it is desirable to employ a transparent plastic material for the substrate 2702. Applicable plastics include, without limitation, polymethylmethacrylate (PMMA) and polycarbonate. When plastic materials are used, it also becomes possible to utilize an injection molding or stamping process for the formation of spacer posts 2708. In such a process the spacer posts are formed in a mold or a stamper first, before the application of the dielectrically enhanced metal mirror 2704. All of the layers of the dielectrically enhanced metal mirror 2704 would be then be deposited in sequence on top of the substrate which already includes spacer posts 2708. The light absorbing layer 2706 is deposited on top of the dielectric mirror 2704. In order to pattern the aperture window 2709 a special photoresist is applied that uniformly coats the surfaces of the thin films without being disrupted by the presence of spacer posts 2708. Suitable photoresists include spray-on photoresists and electroplated photoresists. Alternately, a spin-on resist is applied followed by a reflow step that provides an even resist thickness across the thin film surfaces in the areas of apertures 2709. The exposure of the resist, developing, and etching of the thin film layers then proceeds as described above. After the removal of the photoresist, the process is complete. A liftoff process can also be employed to pattern the dielectrically enhanced mirror as described above. The use of a molding or stamping process for the formation of spacer posts 2708 helps to reduce the material costs required in the fabrication of aperture plate 2700:

In some display implementations, aperture plate 2346 is combined with light guide 2348 into one solid body, referred to herein as a unitary or composite backlight, described further in U.S. patent application Ser. Nos. 11/218,690 and 11/528,191, respectively. Both applications are incorporated herein by reference. All of the processes described above for the formation of the dielectrically enhanced metal mirror 2704, for the light absorbing layer 2706, and/or for the spacer posts 2708 can be similarly applied to a substrate which is bonded to or otherwise indistinguishable from the light guide. The surface of the unitary backlight onto which the thin films are applied can be glass, or it could be plastic, including a plastic which has been molded to form spacer posts, such as spacers post 2357.

In one implementation, the spacer posts 2708 are formed or attached to aperture plate 2700 before the aperture plate is aligned to a modulator substrate, such as modulator substrate 2342. In an alternative implementation of display apparatus 2340, the spacer posts 2357 are fabricated on top of and as a part of the modulator substrate 2342, before the modulator substrate is aligned to the aperture plate 2346. Such an implementation was described with respect to FIG. 20 within the aforementioned U.S. patent application Ser. No. 11/361,785.

FIG. 28 is a cross sectional view of a display according to an illustrative embodiment of the invention. The display assembly 2800 comprises a modulator substrate 2802 and an aperture plate 2804. The display assembly 2800 also includes a set of shutter assemblies 2806 and a reflective aperture layer 2808. The reflective aperture layer 2805 includes apertures 2810. A predetermined gap or separation H12 between the substrate 2802 and 2804 is maintained by the opposing set of spacers 2812 and 2814. The spacers 2812 are formed on or as part of the modulator substrate 2802. The spacers 2814 are formed on or as part of the aperture plate 2804. During assembly, the two substrates 2802 and 2804 are aligned so that spacers 2812 on the modulator substrate 2802 make contact with their respective spacers 2814.

The separation or distance H12 of this illustrative example, is 8 microns. To establish this separation, the spacers 2812 are 2 microns tall and the spacers 2814 are 6 microns tall. Alternately, both spacers 2812 and 2814 can be 4 microns tall, or the spacers 2812 can be 6 microns tall while the spacers 2814 are 2 microns tall. In fact, any combination of spacer heights can be employed as long as their total height establishes the desired separation H12.

Providing spacers on both of the substrates 2802 and 2804, which are then aligned or mated during assembly, has advantages with respect to materials and processing costs. The provision of a very tall (e.g. 8 micron) spacer, such as spacer 2708, can be costly as it can require relatively long times for the cure, exposure, and development of a photo-imagable polymer. The use of mating spacers as in display assembly 2800 allows for the use of thinner coatings of the polymer on each of the substrates.

In another implementation, the spacers 2812 which are formed on the modulator substrate 2802 can be formed from the same materials and patterning steps that were used to form the shutter assemblies 2806. For instance, the anchors employed for shutter assemblies 2806 (similar to anchors 2356) can also perform a function similar to spacer 2812. In this implementation a separate application of a polymer material to form a spacer would not be required and a separate exposure mask for the spacers would not be required.

The display assembly 2900 of FIG. 29A illustrates one methodology for aligning a modulator substrate and an aperture plate, according to an illustrative embodiment of the invention. The display assembly 2900 comprises modulator substrate 2902 and aperture plate 2904. The display assembly 2900 also comprises a set of shutter assemblies 2906 and a reflective aperture layer 2908, including apertures 2910. A predetermined gap or separation between the substrate 2902 and 2904 is maintained by the spacers 2912. The spacers 2912 are formed on or as part of the aperture plate 2904. The display assembly 2900 also includes a set of alignment guides 2914. These alignment guides are formed on or as part of the modulator substrate 2902. When the modulator substrate 2902 and aperture plate 2904 are assembled together, the gap between the alignment guides 2914 and the spacers 2912 is quite close, in some cases less than 1 micron. By capturing the spacer posts between the closely spaced alignment guides, sideways motion between the modulator substrate 2902 and the aperture plate 2904 is restricted, thereby maintaining the alignment between the shutters 2906 and the aperture 2910.

In various implementations, the alignment guides 2914 are ring or doughnut-shaped. In other implementations, the alignment guide 2914 is a simple slot which captures a wall like shape from the aperture plate 2904. The alignment slots can be oriented parallel to either or both edges of the modulator substrate 2903. Having alignment slots with different, and preferably perpendicular, orientations helps to prevent motion in any direction parallel to the plane of the substrate 2902, though other orientations may also be employed The alignment guides 2914 can be placed either between pixels, within pixels, or external to the array of pixels along the periphery of the display.

Alternate means of maintaining alignment between modulator substrates and aperture plates are possible. In one implementation an adhesive, such as adhesive 2128 in FIG. 21, is provided for holding two substrates together in lateral alignment. In this implementation an alignment device, for example, a mechanical platform equipped with translational motor drives and an alignment camera, is utilized to hold the two substrates in their proper orientation while an adhesive, such as adhesive 2128, is dried or cured in place. Epoxies that are partially or totally cured by means of UV radiation are particularly useful as adhesives in this implementation. In implementations where the adhesive is applied at the periphery of the display assembly, it is referred to as an edge seal or gasket seal. In some implementations the edge seal adhesive contains glass or polymer beads which act as spacers for maintaining a predetermined gap or spacing between the opposing substrates.

The display assembly 2950 of FIG. 29B illustrates another means for aligning a modulator substrate to an aperture plate, according to an illustrative embodiment of the invention. The display assembly 2950 comprises modulator substrate 2952 and aperture plate 2954. The display assembly 2950 also comprises a set of shutter assemblies 2956 and a reflective aperture layer 2958, including apertures 2960. A predetermined gap or separation between the substrate 2952 and 2954 is maintained by the opposing set of spacers 2962 and 2964. The spacers 2962 are formed on or as part of the modulator substrate 2952. The spacers 2964 are formed on or as part of the aperture plate 2954.

The spacers 2962 and 2964 are made of different materials. For the embodiment of display assembly 2950 the spacers 2962 are made of a heat re-flowable material such as solder while the spacers 2964 are made of a material which is substantially solid or has a melting or softening point considerably higher than that of spacer 2962. The material in use for substantially solid spacer 2964 can be any of the materials described above with respect to spacer post 2708, including the materials listed in the incorporated U.S. patent application Ser. No. 11/361,785, or it can be any of the materials described below with respect to conductive spacer 3112.

The spacer 2962, also referred to as a solder bump, can be made of a number of different metals or metal alloys commonly used for the soldering of electrical connections. Exemplary alloys include, without exclusion, Pb—Sn alloys, Pb—In alloys, In—Sn alloys, In—Cu—Sn alloys, Au—Sn alloys, Bi—Sn alloys, or the substantially pure metals In, Sn, Ga or Bi. Such alloys are designed to liquefy or re-flow at temperatures in the range of 150 to 400 Centigrade and to wet the surfaces of two opposing contact materials. After cooling and solidification the solder materials join together (and optionally electrically connect) the two opposing contact materials, acting as an adhesive. For the application illustrated by display assembly 2950 the solder material 2962 acts as an adhesive link between spacer post 2964 and the modulator substrate 2952. Non-metallic reflow materials are also applicable for use as spacer 2962. These materials include glass frit materials, such as mixtures of barium-silicate or lead-silicate glasses or thermoplastic polymers, such as polyethylene, polystyrene, or polypropylene, and/or natural and synthetic waxes such as carnauba wax, paraffin, or olefin waxes.

The assembly process for display 2950 would proceed as follows. First the spacer materials 2962 and 2964 would be fabricated onto their respective substrates. Next the two substrates 2952 and 2954 would be assembled together with roughly the correct lateral alignment. Next the two substrates would be heated so as to liquefy or reflow the solder material 2962. Once molten, the material 2962 would proceed to wet the surface of the substantially solid and opposing spacer post 2964. Simultaneously, the surface tension of the now liquid material 2962 will act to minimize its surface area. The resulting capillary forces have the effect of pulling or sliding the two substrates 2952 and 2954 laterally into a more perfect alignment. After cooling and solidification the two substrates are locked into alignment by the adhesive properties of the solder material 2962.

In an alternate implementation the substantially solid material can be fabricated onto the modulator substrate while the heat re-flowable material is fabricated onto the aperture plate. In another implementation both the solid and reflow materials are formed sequentially onto one or the other of the modulator substrate or the aperture plate. For that implementation the solder material is designed to wet and join to a bonding pad located on the opposing substrate.

There are other bonding geometries where a heat re-flowable material can be used to ensure the alignment between opposing substrates. In one implementation solder bumps are fabricated on both of the substrates, and the bumps or beads of solder material are merged or joined during the reflow process. In this implementation a substantially solid spacer material, such as spacer 2964 is not employed. The gap between the two substrates is determined by the average volume of the solder bumps after solidification, which can be controlled by means of the fabricated dimensions for the solder bumps prior to assembly. In another implementation the spacer post 2964 is replaced with a detent or solder receptacle structure, such as a ring or a square frame formed on one of the two substrates with an indent at the center. The molten solder tends to wet and fill the gap at the center of the ring, thereby pulling the opposing substrates into alignment. Similar processes related to the alignment of semi-conductor packages are described in further detail in U.S. Pat. No. 5,477,086, the entirety of which is incorporated herein by reference.

In some implementations the heat re-flowable material is disposed within or between each of the pixels in the array. In other implementations the pairing of re-flow materials to corresponding posts, receptacles, or bond pads on the opposing substrate can be arranged at the periphery of the display or at the outside corners of the display assembly. When the re-flow material is disposed at the periphery of the display it can, in some implementations, serve a further purpose similar to epoxy 2128 (see FIG. 21) as a sealing or gasket material.

FIGS. 30A and 30B are top views of a display assembly 3000, similar to the shutter assembly 2200 of FIGS. 22A and 22B, in opened and closed positions, respectively, according to an illustrative embodiment of the invention. The display assembly 3000 includes a shutter assembly, including shutter 3002, supported over a reflective aperture layer 3004. The shutter assembly also includes portions of opposing actuators 3008 and 3010. In display assembly 3000 the shutter is connected to the modulator substrate (not shown), such as modulator substrate 2342 via the actuators 3008 and 3010 and the anchors 3006 and 3007. The reflective aperture layer 3004 is formed on a separate substrate, such as aperture plate 2346. The shutter assembly 3001 is suitable for inclusion in an array of light modulators included in a display apparatus.

The shutter 3002 includes three shutter apertures 3012, through which light can pass. The remainder of the shutter 3002 obstructs the passage of light. The reflective aperture layer 3004 includes apertures 3014 which also allow the passage of light. In FIG. 30A, where the shutter is in the open position, the apertures 3012 and 3014 are aligned to allow passage of light. In FIG. 30B, where the shutter is in the closed position, the shutter 3002 obstructs the passage of light through apertures 3014.

The display apparatus 3000 includes spacer posts 3020 which function in a manner similar to spacer posts 2357 of display apparatus 2340 for maintaining a predetermined gap or spacing between opposing substrates. The spacer posts 3020, however, provide an additional function of ensuring the proper alignment between the shutter 3002 and the apertures 3014 by fitting between motion stops 3022, which are solid extensions of the shutter 3002. When in the open position, the shutter 3002, via one set of motion stops, comes into hard contact with the spacer post 3020. When in the closed position the shutter 3002, via another set of motion stops 3022, contacts the spacer post 3020 again. The amount of motion allowed for the shutter 3002 between each of the stop positions, in various implementations, ranges from about 5 to about 50 microns.

The alignment control methodology of display assembly 3000 is effective for the case where both apertures 3014 and spacer posts 3020 are attached to one substrate, e.g. the aperture plate, while the shutter 3002 is attached by means of anchors 3007 to another substrate, e.g. the modulator substrate. In such implementations, despite misalignments of as much as 5 or 10 microns during assembly of the two substrates, a proper alignment between the shutters 3002 and the apertures 3014 can be provided. Thus, a shutter/aperture overlap, e.g. overlap W1 in display apparatus 2300, of as narrow as 1 micron can be maintained.

FIG. 31 is a cross sectional view of a display assembly 3100, according to another illustrative embodiment of the invention. The display assembly 3100 includes a modulator substrate 3102 and an aperture plate 3104. The display assembly 3100 also includes a set of shutter assemblies 3106 and a reflective aperture layer 3108. Additionally, the aperture plate 3104 includes a dielectric isolation layer 3114 and an electrically conductive interconnect 3116. The cross sectional view in FIG. 31 is taken along a line where these are no aperture holes in the reflective layer 3108. A predetermined gap or separation between the substrate 3102 and 3104 is maintained by a set of spacers 3112.

The spacers 3112 of display assembly 3100 include an electrically conductive material. For example, they can be formed from metal posts that are formed by either an electroplating process, an etching process, or a lift-off process. The metals copper, nickel, aluminum, gold, or titanium, without exclusion, are useful for this application. Alternately the posts 3112 can be formed from a composite material, for example, a polymer or epoxy material that is impregnated with metal particles to render it conductive. Alternatively the posts 3112 can be formed from polymer materials that are coated with thin metal films such that the surface is made conductive. The electrically conductive spacers 3112 are disposed to provide an electrical contact between one electrode of the shutter assemblies 3106 and the electrically conductive interconnect 3116. For instance, as shown in FIG. 31, the interconnect 3116 can be patterned as a metal line that is parallel to one of the row or columns in the display. The spacers 3112 and the interconnect 3116 provide, then, a common electrical connection between electrodes in all of the shutter assemblies 3106 for the given row or column. In this fashion, some of the metal layers used to form an addressing or control matrix for a display, such as display apparatus 2340, can be fabricated on top of the aperture plate 3104 instead of on the modulator substrate 3102. The spacers 3112 can be configured to provide a distinct electrical connection between the light modulators 3106 within each of the pixels in the array to an electrical circuit on the opposing substrate. In an extreme case each of the electrodes for each of the shutter assemblies can be connected to a circuit on the opposing substrate by means of electrically conductive spacer posts, such that the complete control circuit, including transistors and capacitors, can be formed on a substrate that is distinct from or separate from that of the light modulators.

The electrically conductive spacer posts 3112 can be formed onto either the modulator substrate 3102 or the aperture plate 3104 prior to assembly. In one implementation the conductive spacers are formed from gold-alloy studs that are individually placed and bonded by machine onto one or the other of substrates 3102 or 3104 as part of the assembly process. In another implementation the spacers are formed from solder bumps which are either electroplated or stenciled onto one or the other or both of substrates 3102 or 3104 as part of the assembly process. Any of the solder materials listed above with respect to solder bump 2962 can be employed for spacer posts 3112.

The process of matching or making electrical connection between a spacer posts and a landing zone on the opposing substrate can include a reflow process, such as an interdiffusion or a soldering process, so that a good electrical contact is made between the two surfaces.

Although the display assembly 3100 is illustrated as part of a MEMS-down configuration, an analogous use for electrically conductive spacer posts can also be found in the MEMS-up configuration as with, for example, the display apparatus 2300. For a MEMS-up configuration both the light modulators and the reflective aperture layer are formed on the same substrate, while the electrically conductive interconnects, such as interconnects 3116 are formed on an opposing substrate, such as a cover plate. Conductive spacers, such as spacers 3112, would then make electrical connection between the modulator substrate and the cover plate. The invention is particularly useful for applications where the cover plate also acts as a touch-screen input device. Electrical connections between substrates, especially where connections are provided for each pixel in the array, are useful for providing electrical communication between a touch screen sensor array and a separate substrate that includes a control matrix for the array of pixels.

FIG. 32 is a cross sectional view of another display assembly 3200, according to an illustrative embodiment of the invention. The display assembly 3200 includes a modulator substrate 3202 and an aperture plate 3204. The display assembly 3200 also includes a set of electrical interconnects 3210 formed on the modulator substrate and a set of electrical interconnects 3212 formed on the aperture plate 3204. The two substrates make electrical connections between the interconnects 3210 and 3212 by means of an anisotropic conductive adhesive 3214. The aperture plate also includes a dielectric isolation layer 3211 and a reflective aperture layer 3208. Not shown in this view are shutter assemblies formed on substrate 3202 or any aperture windows in the reflective layer 3208, since this cross sectional view is taken along a line closer to the periphery of the display, outside the array of pixels.

The anisotropic conductive adhesive (i.e. “ACA”) 3214 is a polymer adhesive that is impregnated with a collection of solid conducting spheres 3216. When the two substrates 3202 and 3204 are compressed together the conducting spheres become trapped between the contact areas of interconnects 3210 and 3212. An electrical connection is thereby established between the interconnects 3210 and 3212 on opposing substrates. The electrical connection becomes locked in place after the polymer matrix of the ACA is cured or polymerized. The conducting spheres 3216 can range in size from 5 to 50 microns in diameter. The spheres 3216 can be made of a metal, such as nickel, or from alloys such as Ni—Au, Ni—Ag, or Cr—Au. Nickel has sufficient hardness to maintain it's geometry under compressive loading. The conducting spheres can also be fabricated from dielectric materials, such as glass or polymer, which are then coated with a conducting layer such as gold. In an alternate embodiment, metal spheres such as nickel can be coated with another metal with a higher conductivity and resistance to oxidation, such as gold, to reduce contact resistance.

When the conducting spheres are selected for uniform diameter, then the squeezing of the contacts 3210 and 3212 to the conducting spheres 3216 establishes a fixed spacing or gap between the contacts. The conducting spheres 3216 can therefore perform the same function as the spacers 2312 or 2357.

The conducting spheres do not make a continuous contact along an axis parallel to the plane of the substrates. As a result, an electrical connection is generally not established between neighboring pairs of conductors 3210 or pairs of conductors 3212. Therefore a single application of the ACA 3214 can be sufficient to make multiple independent electrical connections between independent sets of interconnects 3210 or 3212. This particular interconnection medium is referred to as an anisotropic conductive medium.

The ACA 3214 is generally applied by means of a tape or needle dispense over multiple independent electrical contacts. It is particularly useful for making multiple connections along the periphery of a display. For instance it can connect together a series of row interconnects on one substrate to a parallel set of row interconnects on another substrate. This might be useful, perhaps, where the control matrix is built onto the modulator substrate 3202 while driver chips are attached to a parallel set of interconnects on the aperture plate 3204. The functions of the substrates 3202 and 3206 can also be reversed, so that the modulator substrate is on the bottom, closest to the backlight and the substrate on the other side of the ACA 3214 functions as a cover plate.

The electrically conductive spacers 3112 in display assembly 3100 are preferably used in each and every pixel in the array, although they can be applied on the periphery of the array. The electrically conductive spheres 3216, which are also spacers, in display assembly 3200 are preferably applied along the periphery of the array of pixels. The spacers 3020 of display assembly 3100, which perform as motion stops, are preferably used at every pixel in the array. The spacers 2312 or 2357 of display apparatus 2300 and 2340, respectively, can be placed either within or between each pixel of the array. Similarly the spacers depicted in FIGS. 20 and 21 of U.S. patent application Ser. No. 11/361,785, may also be included with each and every pixel in the array.

The association of a spacer with each and every pixel is not, however, necessary to maintain a desired gap between substrates. A spacer can be associated, for instance, with each group of four pixels or with each group of 16 pixels. In other embodiments the spacers might be restricted to the periphery of the display. Alternatively, as depicted in display assembly 3000, a display assembly can include multiple spacers per pixel.

The spacers 2357 in display apparatus 2340, for instance, are responsible for maintaining an aperture to shutter spacing H6 that can be as small as 1 micron. Denser spacer placing becomes particularly useful when display resolutions exceed 200 pixels per row or column, or when display diagonals exceed 2 inches. A dense array of spacers is also useful in display apparatus 2300, for example, for maintaining a uniform pressure on the lubricating fluid 2322, which might otherwise be disrupted by local pressures, such as finger pressure, applied to the front surface of the display.

The spacers described with respect to this invention can be usefully applied to maintain the gap between substrates in an electrowetting display, such as display apparatus 2500 or 2600. Any display that employs MEMS-based light modulators, in fact, can benefit from spacers applied within the interior of the array of light modulators. Examples of alternate MEMS-based light modulators include digital mirror devices (DMDs), interference modulation displays (IMODs), and light tap displays or frustrated internal reflection displays.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The forgoing embodiments are therefore to be considered in all respects illustrative, rather than limiting of the invention. 

1. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image; and a light guide positioned behind the first substrate, wherein the reflective aperture layer reflects light not passing through the plurality of apertures included in the reflective aperture layer back towards the light guide.
 2. The display apparatus of claim 1, wherein the first substrate comprises a light guide.
 3. The display apparatus of claim 1, wherein the plurality of MEMS light modulators comprise shutter-based light modulators.
 4. The display apparatus of claim 1, wherein the plurality of MEMS light modulators comprise electrowetting light modulators.
 5. The display apparatus of claim 1, wherein the MEMS light modulators are disposed on the first substrate.
 6. The display apparatus of claim 1, wherein the first substrate is transparent.
 7. The display apparatus of claim 1, wherein the first substrate is in intimate contact with the light guide.
 8. The display apparatus of claim 1, wherein the reflective aperture layer is formed from one of a mirror, a dielectric mirror, and a metallic film.
 9. The display apparatus of claim 1, wherein the first substrate is positioned such that the reflective aperture layer is proximate the plurality of light modulators.
 10. The display apparatus of claim 1, wherein the first substrate is positioned with respect to a light guide so as to form a gap between the first substrate and the light guide.
 11. The display apparatus of claim 10, comprising a fluid filling the gap.
 12. The display apparatus of claim 11, wherein the fluid has a first index of refraction and the light guide has a second index of refraction, and wherein the first index of refraction is less than that of the second index of refraction.
 13. The display apparatus of claim 1, wherein the plurality of MEMS light modulators are formed on the second substrate.
 14. The display apparatus of claim 13, wherein the plurality of MEMS light modulators are formed on a rear-facing surface of the second substrate.
 15. The display apparatus of claim 1, comprising a control matrix formed on the second substrate for controlling the plurality of MEMS light modulators.
 16. The display apparatus of claim 1, wherein the second substrate is transparent.
 17. The display apparatus of claim 1, comprising mechanically interlocking features for maintaining lateral alignment between the first and second substrates.
 18. The display apparatus of claim 1, comprising an adhesive for maintaining lateral alignment between the first and second substrates to less than 5 microns in any dimension.
 19. The display apparatus of claim 18, wherein the adhesive comprises a heat-reflowable material.
 20. The display apparatus of claim 1, wherein the first substrate is positioned with respect to the second substrate so as to form a gap between the first substrate and the second substrate, the display apparatus comprising spacers for maintaining the gap.
 21. The display apparatus of claim 20, comprising a fluid filling the gap.
 22. The display apparatus of claim 21, wherein the fluid is a lubricant.
 23. The display apparatus of claim 21, wherein the fluid has a first index of refraction and the substrate has a second index of refraction, and wherein the first index of refraction is greater than or substantially equal to that of the second index of refraction.
 24. The display apparatus of claim 20, comprising a liquid filling the gap.
 25. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image, wherein the first substrate comprises a light guide.
 26. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image, wherein the plurality of MEMS light modulators comprise electrowetting light modulators.
 27. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image, wherein the first substrate is positioned with respect to a light guide so as to form a gap between the first substrate and the light guide.
 28. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image; and mechanically interlocking features for maintaining lateral alignment between the first and second substrates.
 29. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image; and an adhesive for maintaining lateral alignment between the first and second substrates to less than 5 microns in any dimension.
 30. The display apparatus of claim 29, wherein the adhesive comprises a heat-reflowable material.
 31. The display apparatus of claim 30, wherein the lubricant has a first index of refraction and the substrate has a second index of refraction, and wherein the first index of refraction is greater than or substantially equal to that of the second index of refraction.
 32. A display apparatus comprising: a first substrate having a front-facing surface and a rear-facing surface; a second substrate in front of the front-facing surface of the first surface, wherein the first substrate is positioned with respect to the second substrate so as to form a gap between the first substrate and the second substrate, the display apparatus comprising spacers for maintaining the gap; a reflective aperture layer including a plurality of apertures disposed on the front-facing surface of the first substrate; a plurality of MEMS light modulators for modulating light directed towards the plurality of apertures to form an image; and a fluid filling the gap, wherein the fluid is a lubricant. 